Differential swap out options

[Montierth]

I currently have a 1970 GMC with leaf springs, 5 lug, disc brakes, non posi, 12 bolt differential with 3.08 gearing. I am interested in having a 3.73 gearing with posi/limited slip and keeping the 5 lug and disc brakes. I have looked into different options and seems like easiest would be to find a used differential that is already 3.73 and posi. Also, in the future I would like to convert to trailing arms. I have read that somewhere around 1970 the GMCs differential was a little different size than the Chevys?

My question is what would be a simple bolt in? Are there certain years that work better?

I found a 1998 10 bolt 3.73 posi differential someone is selling but I would have to buy 5 lug axles. Not sure though if it would fit.

[Pep01]

I have a 69 with leaf springs. Also interested in this answer.

[71meangreenc10]

Mid year 1970 the axle length is longer than the early model. You can pull 5 lug axles from 71 and up and slide them right in to replace the 6 lugs. Trailing arm conversion is going to be a little more interesting to complete. If you want to change the diff, look for a 12 bolt in an early square and bolt it in. You do have to fix the shock mounts locations/possible park brake cable replacement. Axles can move between coils and leafs, but it is not just a bolt in. Mounting perches have to be done and adding or subtracting a panhard bar.

Best option to me is to re-gear your current axle. You have a lot of shops near you unlike me (I do it all myself). I'd pull the axle, clean it and drop it off. 3.73 from 3.07 will have the Gas Gauge doing the Electric Slide faster than it used to do it.

No matter which way you go now, the axle issue is going to come up twice to solve if you do a suspension change.

Smitty

[Montierth]

Thanks for the reply. I realize it will be work with relocating perches, shocks, possible drive shaft length, etc. You are probably right about just regearing. I have thought about regearing and posi myself - I have done everything else on my truck. Read a lot and watched a lot of videos on how to DIY and there would be a few areas of challenge for me as well as just screwing it up and now out a rearend. I think it would be funner to go with the posi/3.73 gearing but really not necessary and find I can easily cruise on the freeway going to Dino's Git Down going 75-85 mph keeping up with crazy fast AZ drivers! So I guess what I mean is I don't want gear changers remorse. Almost would like to have a 2nd differential so if I ever wanted to go back to the 3.08 it would just be my time.

I slid under the truck and got a rough measure of 59 inches from the inside of the mounting plates. Would an early square bolt right in if I kept leaf springs?

Originally Posted by 71meangreenc10

Mid year 1970 the axle length is longer than the early model. You can pull 5 lug axles from 71 and up and slide them right in to replace the 6 lugs. Trailing arm conversion is going to be a little more interesting to complete. If you want to change the diff, look for a 12 bolt in an early square and bolt it in. You do have to fix the shock mounts locations/possible park brake cable replacement. Axles can move between coils and leafs, but it is not just a bolt in. Mounting perches have to be done and adding or subtracting a panhard bar.

Best option to me is to re-gear your current axle. You have a lot of shops near you unlike me (I do it all myself). I'd pull the axle, clean it and drop it off. 3.73 from 3.07 will have the Gas Gauge doing the Electric Slide faster than it used to do it.

No matter which way you go now, the axle issue is going to come up twice to solve if you do a suspension change.

Smitty