1970 GMC 2wd to 4wd conversion

[caseymp2001]

I have a 1970 2wd half ton short box. I want to make the truck a 4wd. I want to run a solid front axle. I need to know if someone was going to do this what axles they would run. or should I just sell the truck and start fresh?

[dgfreeze]

i did it to mine, but it was a LOT of work, because i went from a 305 V6 with a manual tranny to a SBC 350 with a th350 and 205 transfer case. it was a lot of fun at times, but holy crap. oh, i'm not saying this is "the way" to go, but i have a 10 bolt front axle, and 14 bolt rear. i had to custom fab all the crossmembers, and all but one of the spring perches.

[justcuz]

Your best bet would be to find a shortbed 4x4 frame and swap over. If you can't do that then look for a donor frame and just swap over the 4x4 stuff to your chassis. Most of the holes are pre-drilled and it is a lot of work, but if done carefully works out fine. I am doing this to my 1973 Suburban using parts from a 74 Blazer frame. I would recommend 2 improvements during the swap. Box plate or reinforce the factory engine crossmember. Add an aftermarket steering box brace. Also if your truck is a stick, (and your are swapping in an automatic) leave the bell housing crossmember in, it adds stiffness to the chassis. I will probably make mounts for mine that use the two lower auto trans mounting bolts and drop down to this crossmember. That way I do not have to install the rods that go from the engine mounts back to the bottom of the bell housing that tie everything together (which does not strengthen the frame, the crossmember does). Personally I prefer 3/4 ton axles, however if you stay half ton 6 lug, find a semi-float 14 bolt out of a square body and swap out the 8 lug axles and drums with 6 lug axles and drums. There are 3 width axles on the semi float, the widest is from an H2 Hummer I believe they may be 8 lug only. The other 2 are from light duty 2500 trucks 88 and up. The 4x4 axles are 1.5 inches longer than the 2wd axles. I think the 2wd axles are the same length as all the square body 8 lug axles. Measure to be sure. I think wms to wms on the 2wd SF is the same as all square body 8 lug SF. These have 35 spline axles and will last forever in the back of a 1/2 ton shortbed. They are rated as a 5000 lb axle. The front axles in the six lug are pretty much the same strength whether it is a GM 10 bolt or a Dana 44. They use the same outers from the knuckle out and I think some of the axles interchange. For eight lug stuff you have a choice of the 10 bolt, Dana 44 or a Dana 60. A Dana 60 is overkill unless you are gonna wheel it to death or plow with it.

[Gene]

I am halfway converting my 2wd truck to a 4wd, I got the front done using DIY4X front brackets, but I kept searching and found a short wheelbase 4wd rolling chassis on craigslist locally, they are out there believe it or not. But I still plan on finishing the frame I started with as soon as I get the bucks to finish it up. You can do the front easily with the diy4x brackets, and the back is really not that big a deal either if you can find a 2wd GMC with leafs(has to be a 2wd because of the difference in widths in the rear where the coils used to mount).

[justcuz]

I converted a 65 Chevy from coils to leafs, and went from 1/2 to 3/4 ton. No big deal at all. Same frame used 73 to 87 2wd spring mounts.

[Gene]

So a 2wd 73-87 w/leafs is the same dimensions as a 67-72 w/coils? I did not know they would work??

[justcuz]

Ahead and behind the neck in on the frame (on the coil frame), the widths are the same, I forget how I lined it all up. I think a hole or two acted as a pilot for the rear shackle bracket. I worked forward from there, setting the approximate shackle angle with the axle centered in the wheel well and set the mounts for the front brackets. The truck was involved in n accident that took out the front clip and bent the frame horns back to the crossmember under the engine. I sold the cab, used the engine in my Chevelle, still have the truck 4 spd and rear axle. I used the rest to make a trailer. I narrowed a boat trailer axle for the trailer. I can take pictures for you of the brackets if you are interested.

[caseymp2001]

so would a frame from a 1980 to 1985 1 ton 4x4 short box work for this ?

[justcuz]

What is the wheelbase? If it is the same as a short bed early truck, then probably. You may have to change cab mounts but I bet the bed mounts are the same. The way the core support is mounted looks close. See if you can find a Mitchells repair guide online that has the frame measurements. They are called crash sheets or datum sheets for frames. Look up the years you want to swap and compare measurements. I do know from 73 up the 4x4 front springs are longer. That should not change the frame or wheelbase I don't think.

[iluvmy72]

doing this kind of swap scares me in the equipment alone....you'll need two different welders, a plasma cutter, grinder, air tools, and a bbq for your lunch. Get a different frame. The shackle alignment ia enough to kill you.

[Gene]

Jimmy/Blazers are shorter wheelbase than a truck, trucks are 115 and I believe the jimmy/blazers are 104? I guess you could stretch the frame? I would look for a longbed 4x4 and cut it down before I would try stretching one.