700 R4 swap 72 K10

[Nitis]

I have a 72 K10 that I am trying to decide what direction to go with I am a firm believer in overdrives and am still seriously considering an LS swap which would constitute no need for this conversation

But I think I could more easily justify building a new engine now pairing it with the current turbo 350 then later add the overdrive tranny

So what mods could one expect going from TH350 to 700r4? Will I need new drive lines cut or lengthened? What else will I have to do?

Sorry not a mechanic just trying to get an idea I know overdrive will help make the truck more driveable and squeeze some mpg out of it too

[Ziegelsteinfaust]

There are 2 ways to do it roughly. Or 2 I looked at when I was planning my truck as I had a chance to swap for a 4L65, and a NP208.

1. Buy adapter for a 700R4 to your transfer case, move your assembly back a couple of inches, get new drive shafts or have yours adjusted.

2. Buy a NP208 or NP241 with the 700R4 transmission, move everything back, and get new driveshafts made or your adjusted.

Which is why on my truck the working plan is now too put 3.42 gears in my axles, and get my TH350 rebuilt with a wide ratio kit. Also when the tranny gets rebuilt have a few upgrades done to increase strength. My tires will either be 32 or 33 inches tall, and I will have a highway rpm of roughly 2500 at 70.

My current 350 has bolt ons, and stock cam. So torque should be more then enough to power her down the road, and take off with a 6000 pound trailer. Eventually I may up it to a 350 Vortec with a cam or a LT1 to gain the roller cam, and power.

Another idea I has was to use the 4.11 gears, and build a 327 or 302. Both of which are more rev friendly on mpg, and the deeper gearing makes up for the lack of torque.

[Nitis]

Well I guess gears are another option. I guess I should probably just get on with getting a new engine and then see what I think

I plan to run 33s and will install a new 2.5" lift.

Ya know for 90% of where I go in this truck I wouldn't ever use the overdrive. But if I had the overdrive I would likely drive it more places. And this thing needs to be driven they weren't built to sit pretty

[Ziegelsteinfaust]

I have a K20, and I have to thought about OD, but when I got right down to it I would never get enough of a increase in MPG. My belief is if I set up my truck properly with my stock old school 350 I will get 14-15 mpg highway, and with OD probably get 17-18 mpg hwy tops. It would have broader performance, but cost well over $2500 to do, and cost me several weekends of time. For about $1300 installed I could have my TH350 rebuilt with a wide ratio kit, and a 2000 rpm converter to help get me off the line. Plus still have well over $1200 to invest into a LT1 or Vortec 350 to increase low speed torque, and higher speed power.

For you LS guys I am bought into SBC's, and don't feel the need to change anytime soon.

Lets say you have a fresh 350, and are running AT tires. The tires pose a decent amount of drag to you mpg that the extra rpm of no OD is not a penalty. So spinning 2500 with 3.42's or 2700 with 3.73 gears with 33" tires is almost necessary to keep the truck moving easily.

Also new trucks in 4x4 form with AT tires probably get 17mpg cruising about 70, and more aggressive tread will hurt it more. So if your old truck properly set-up gets 15 which is my goal I am not to far behind to care. The new truck will have broader performance with more gears, but not necessarily more hwy mpg.

[ryanroo]

The OD will make the truck more driver friendly. if you bolt a 700r4 behind your 350, it will be about 2-2.5" longer. that will require a shortened rear driveline and lengthened front. another option used by many is moving the engine forward to accommodate the length. that way you can use the driveline as is. having done a couple of conversions to OD, i recommend it. if i had an engine that was in decent shape to begin with my first step would be the OD.

[Bigdav160]

I'm with roo. You either have to move the tranfer case crossmember back or move the engine crossmember forward.

I'd move the engine. Easier than building new shafts

[derotoreut]

Originally Posted by ryanroo

The OD will make the truck more driver friendly. if you bolt a 700r4 behind your 350, it will be about 2-2.5" longer. that will require a shortened rear driveline and lengthened front. another option used by many is moving the engine forward to accommodate the length. that way you can use the driveline as is. having done a couple of conversions to OD, i recommend it. if i had an engine that was in decent shape to begin with my first step would be the OD.

Ryan - Sounds like you have experience in moving the engine to the forward mounting position. Is that true? You can move it frontward with no problem? I'm wondering why GM used the rearward mounting position for these K series trucks. It would be so much easier to work on them with the engine moved forward to begin with. My C20 seems to have so much more room because of forward engine positioning.

[ryanroo]

That is kind of how it reads, but the recommendation i was making was to go for the OD. personally i do not prefer the engine forward method. its generally a step backwards any way you slice it in terms of performance. so for me i would rather budget the cost of a retube for the front and shorten for the rear driveline. all of my swaps to OD came in the form of NV4500s so i dont have any personal experience with a 700r4 in one of these.

i cant say as i know the exact reason the K series is further back, but my first guess is clearance on the diff. second thought would be a more friendly front driveline geometry.

[freakshow108]

I put a 700R4 behind my 350, in my 71 K20. I chose to move my engine forward. It is very simple and everything lines up. I'm running 4:11's with 35's and at 60 mph I am turning 2400 rpms. Hope this helps