1979 3+3 GMC Dually
 

Hey everyone. I was on here a while back with a 1980 c10, but the project was too big for me while I was in school and working.
Finally picked up a truck I’ve been trying to find for a long time. It’s a 1979 crew cab dually. It’s rust free from California (I’m in MA, everything is rusty) and it seems to be all or mostly original and unmolested.

My plans are pretty simple, being such a nice example I want to leave it mostly alone but clean it up (especially interior), fix the dents, AC, paint the fender etc. then lower it (maybe 6/8?) and put it on 22” semi wheels.

I’ll have some questions for sure, but figured it should go in the project section.
My first question is how to tell what’s original, what the truck came with, paint code, etc. can that all be accessed somewhere with the VIN? I want to check my rear end gears, make sure I get the right grille, etc. and find out any cool info - like what makes it a camper special and that kind of stuff.

On to the pics:

Re: 1979 3+3 GMC Dually
 

Ugh sorry some of them posted sideways. Not sure why lol

Re: 1979 3+3 GMC Dually
 

Beautiful! There should be a spid label in your glovebox or on the fender well under the hood that will label all options. If that is missing, then it will be hard to know all the original options. But You might be able to give the GM heritage center your VIN and maybe they can get you a build sheet.

Re: 1979 3+3 GMC Dually
 

Looks like a decent start. Check the glove box for the SPID info. The label was there for my dually.

Re: 1979 3+3 GMC Dually
 

Dang, no label in the glovebox. Ill see what I can find from the VIN. I'll need the paint code at least to paint the new fender.

What're you guys doing for highway speeds? I assume they all came with 4:10s? 60MPH is pretty high RPM....not great for around here when 80 is normal cruising speed. If I change the gears will it get out of its own way around town?

Ordered some interior parts I needed from Brothers. Of course the dash pad I really need is on backorder.

Next phone call will be for semi wheels and a drop....anyone to stay away from? I've heard good things about lowboy so far.

Re: 1979 3+3 GMC Dually
 

does anyone sell the fender trim? havent seen it online anywhere.

Re: 1979 3+3 GMC Dually
 

Lowboy is mainly bagged set-ups. I asked if he did an arm set-up that would work w/coils & he said "no, but it might be worth looking in to...". Not sure it is on his 'to-do' list but I'm betting no.

3" spindles & drop coil up front get's your 6" drop. The front x-member gusset bars will be creeping in close to the tarmac on decent size dips on the road w/the stock stuff that low.

Flip kit for the rear is 8" (+/- depending on the spring-pack count). At 8" drop you'll be bumpin' the bottom side of the bed floor w/the differential (they're porky).

Nothing should hang below the rails @ 6/8. They'll high center around town.

I think 4.10's is the most common for the HD rears. Mine does around 2800-3k @ 65-70 w/31.7" rear tires. Some guys will do 3.73's & there are also 3.54's available but like you mentioned it could impact the ability to get a load moving. I search C-list weekly for a 4l80e.

Re: 1979 3+3 GMC Dually
 

They came with 3.73, 4.10 or 4.56. Tires came as a 215/75r-16 or a 235/85r-16. 235s are preferred as they are 31.5'' tall. Lowered dually in MA will be fun in the winter

Re: 1979 3+3 GMC Dually
 

I called them yesterday and the guy Eric was really helpful. I ended up ordering a 5/7" kit from them to avoid having to cut into the bed, and our new england roads SUCK so he suggested thats a good height for looks and still keeping some driveability around here.

He also set me up with the semi wheels so I'm real excited to get this stuff in and transform this truck!

I definitely need to decide on something to make it more highway speed friendly. Whats needed on the 4l80e swap? I would assume swapping gears is easier but maybe not?

Re: 1979 3+3 GMC Dually
 

The tires on there now are 29.3" tall and thats right about what the 22" setup will be at also (225/75 R16 and 265/35 R22 ) Hopefully that doesnt hurt me too much. I gotta get the RPMs down on the highway.
Lol, this truck is too nice underneath (and 2wd) to be driving around here in the salt and snow....itll be a 3 season truck only. Ill keep my f250 for winter duty.

Re: 1979 3+3 GMC Dually
 

Taller tires = rpm drop just like putting in different gears.

225/75-16 @ 29.3" w/4.10's 1:1 final drive gear @ 65mph = 3056rpm
225/75-16 w/3.73's & th400 1:1 final drive gear @ 65mph = 2780
265/35-22 w/4.10's & th400 1:1 final drive gear @ 65mph = 3056

265/40-22 @ 30.3" w/4.10's 1:1 final drive gear @ 65mph = 2955rpm
265/40-22 w/3.73's & th400 1:1 final drive gear @ 65mph = 2689

265/45-22 @ 31.3" w/4.10's 1:1 final drive gear @ 65mph = 2861rpm
265/45-22 w/3.73's & th400 1:1 final drive gear @ 65mph = 2603

29.3" tire w/4.10's & 4l80e .75 final driver gear @ 65mph = 2292rpm
29.3" tire w/3.73's & 4l80e .75 final driver gear @ 65mph = 2085

30.3" tire w/4.10's & 4l80e .75 final driver gear @ 65mph = 2216rpm
30.3" tire w/3.73's & 4l80e .75 final driver gear @ 65mph = 2016

31.3" tire w/4.10's & 4l80e .75 final driver gear @ 65mph = 2146rpm
31.3" tire w/3.73's & 4l80e .75 final driver gear @ 65mph = 1952

A Gear Vendors OD set-up would be similar gearing behind a Th400 @ .78 vs .75 final drive ratio (the GV would be slightly higher rpm's vs the 4l80e).

Re: 1979 3+3 GMC Dually
 

I daily drove my swb with T400 and gear vendors for the better part of a decade. Loved that combo.

Also your dark brown is most likely cordovan brown. Light tan might be Sante fe tan.

Re: 1979 3+3 GMC Dually
 

I looked into one for mine. The TH400 was rebuilt prior to my ownership but works fine & has a firm shift. I figured the GVOD would keep things simple. But, the way my carrier bearing & exhaust (3" dual w/H-pipe) is laid out would likely require re-working them to put one in. Plus, w/it being aggressively lowered, the actual GV sump would be near mid-cab which is around the most vulnerable spot in the length of the truck. When your ~5" off the ground w/a ~20' frame length, that's an expensive ouch if it tags anything.

For the cost of each, I'd do a 4l80e in a CC lowered dually.

Re: 1979 3+3 GMC Dually
 

Haha, well when you put it that way, I agree with you 100%!

Re: 1979 3+3 GMC Dually
 

I don't think I saw it mentioned, and from what I see of your engine pic it looks to be a big block, but could you confirm what your truck has for a motor?

Re: 1979 3+3 GMC Dually
 

yes, 454