‘67 C10 Going from sm-420 to TH350
 

I’ve had my truck for a long time and I am a fan of the sm-420, but it’s time to put in an automatic for improved driveability.

My question is:

Besides swapping the cross members and drive shafts, and replacing the clutch / flywheel for a flex plate, what more is needed?

PS it already has an auto tilt column from an ‘81 van.

Not that they show parts relevant to the question, but everyone likes pics, so here you go:

Thanks.
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Re: ‘67 C10 Going from sm-420 to TH350
 

New Radiator with trans cooler, and vacuum line.... Your shop is too clean. looks photo shopped.

Re: ‘67 C10 Going from sm-420 to TH350
 

Haha, thanks.

That photo was taken days after we moved into our house. The previous owner used the detached garage as a ceramic shop, it was meticulous. I like to think of my '67 as a bull in a ceramic shop.

When we first moved in I broke in my new engine (with open headers). A risky move being the new guy on the block, but I lucked out and all the folks were cool with it.

Its funny how having a cool old c10 was the perfect ice breaker to get to know my neighbors. That was years ago, and one of my neighbors just passed away last week. He was an elderly man and a fountain of knowledge on sbc's and old vehicles. If I didn't have my '67, I may never have gotten to know him. He's sorely missed already.

Thanks again for the info.

Re: ‘67 C10 Going from sm-420 to TH350
 

Have you thought about installing a 700R4/4L60? I'm running a TH350 with 3.08 gears and am pulling too many RPMs on the highway, so I just got a used '92 700R4 and dropped it off at the rebuilder. If you're going through the trouble to swap to an automatic, you might consider a 4-speed for the extra gear on the interstate.

Re: ‘67 C10 Going from sm-420 to TH350
 

I’m in the same boat right now with my 67. Swapped engines a few years ago and never finished putting it all back together until recently. Been working on it every night buttoning up all the loose ends. Finally mid day today I fired it up, open headers, and realized I’m “that guy” now. Hate being loud but felt so good firing it up!

Neighbor right behind me on the other side of our alley is a machinist as well, and is always working on his boat and cars but he’s never in back. Three houses down the alley is a dude with a dialed Orange C10 but I’ve never seen him outside, just see it parked or him driving around.

Re: ‘67 C10 Going from sm-420 to TH350
 

I haven't given those options any thought only because I really don't know much about them, and I have a TH350 sitting in my storage unit ('94 cargo van sitting in my brother's horse pasture).

I'm definitely open to alternate / better options. I assume those trannys can be found in the mid '90's trucks?

What should I look for / look out for?

My goal is to be able to drive my truck as much as possible, in town, and on the highways.

Its a c10 with a sbc383 (xe274h cam) with an eddy performer RPM intake, Holley 600cfm, ported heads, and headers. So yes, lower RPM's on the interstate would be great. On that note, what would be a good rear end for this application?

Re: ‘67 C10 Going from sm-420 to TH350
 

Keep firing that thing up, and they will come....

Re: ‘67 C10 Going from sm-420 to TH350
 

There are some good threads on here about 700R4 transmissions. Based on what I've read, the best are later '87-92 units, or a '93 4L60 (renamed from 700R4). For '94 and up GM went with the electronic 4L60E, which would require additional transmission control unit (TCU). Here's a good FAQ:
http://67-72chevytrucks.com/vboard/s...ad.php?t=33266

...and a couple of references for gear ratios and dimensions:
https://mechanicbase.com/transmissio...-transmission/
https://maliburacing.com/auto-tranny-id/

Re: ‘67 C10 Going from sm-420 to TH350
 

Now I have some reading to do. Thanks.

Re: ‘67 C10 Going from sm-420 to TH350
 

Bet you have to swap diff gears

Re: ‘67 C10 Going from sm-420 to TH350
 

Good idea to check the rear gear ratio. The TH350 may be OK depending on the tire size and rear gears and what Crazy is looking to gain. Here's an RPM calculator:
https://spicerparts.com/calculators/...rpm-calculator

For my goals and driving style, I need overdrive. After the 700R4 is installed, I should be cruising at 70 MPH while only pulling just under 2000 RPM. The 700R4 has a lower 1st & 2nd gear than the TH350, so local driving will still be fun.