Here's the scenario. I found a complete '83 3/4 ton 'burban with 6.2/700R4 combo, runs good, etc. for skinny $$ because there's no title. I can use the sheet metal, buckets, console and A/C system to spruce up the dually, but thinking about putting the 6.2/700R4 into my '68 short 1/2 ton. That truck currently has a gas guzzling (11mpg) 350/400 combo with a 3.08 rear end.
For optimum fuel economy, and driveability, what rear end gears would some of you look at running? 3.42, 3.73, 4.11.
I'm assuming I'll have to find a set of big block frame perches, unless I can modify those that come with the 'burb to fit the earlier frame.
I figure on a fuel cell mounted under the bed, and rigging a return line of course.
Aside from all of that, any major problems that any of you can forsee trying to do this swap? Thanks for any thoughts........

I would leave in the 3:08, why change?
My donor vehicle was a 83 Blazer w/3:08 gears and the guy I bought it from said it was good on fuel.
Merle

I'm sure that with 3.08 gears it would be good on fuel, but running the NA 6.2 with an automatic, could it be a bit of a dog off the line? I'm not trying to win any races, but dont want to slow down traffic either.

i'd put it in and try it b4 i spent $$ on a gear you can always swap them later if you don't like it the diesel will bolt up to the s/b mounts and the s/b exhaust will also line right up get a big block rad or bigest rad you can fit in i don't think the diesel one will fit in that truck

Depends on what tire size you want to run too.

3.08/700 combo would be best for mileage, unless you want to run oversize tires, then I would go with 3.42s, but definitely nothing higher.

700 tranny has really low first gear, so getting off the line should still be OK with stock tires.

I had a 6.2/700/3.42 blazer with 32" tires, and it would JUMP off the line!
so if you went 30" tires with 3.08s I think you'd be very happy with it.

Keep the 3.08,
I run 3.42 with 700r4 and 33's.
works fine.

Depends what kind of driving you are planning on doing. Personally the best performance mileage I have seen has been in 3/4 ton trucks with 4.10, 700r4 and 235/85/r16 tires. This combo usually turns in about 20 mpg empty and has good power to pull hills and a trailer. Currently I just put 4.10 in my Daughters Jimmy with 35 inch tires and it is still a littly doggy in overdrive. But the Jimmy scales at 5900 lbs empty.:eek: It runs really good with the 235/85/16 tires.

Thanks for the ideas. I think I'll keep the 3.08 for now. The stock tire are right about 29" tall (235/75-15's)but I suppose I could throw a set of 265's on the rear for a little extra height to experiment with later on. The truck scales in at 3600 lbs with the small block, havent looked up the weight of the 6.2 yet to see how much heavier it is than the small block. Not looking to do any towing or heavy lifting with this one, thats what I got the dually for. Just trying to build a reliable, economical to run, comfortable road trip cruiser.
btw: pics of all the "kids" are at www.picturetrail.com/yachtcare

Real nice dually. that 68 pickup is going to be great when shes done, good luck on it.

When I blew my 350 in my 1970 Chev C20 with a 350 Auto and 4.10 rear end I just replaced it with a new 6.2L engine for engine. I did have to put a vacuum pump on the 6.2L for brakes plus the control valve for the auto to work. I put a different flex plate on the 6.2L plus I think I put a 400 Auto bell housing on the front of the 350 Auto for (starter clearance). When the 350 Auto finally failed then I replaced it with a 3 speed (plus granny) manual trans. I had to modify the drive line when I installed the manual. The 400 Auto is much better than the 350 Auto and I had several friends that replaced their 700 auto with 400 units when they failed. When I put the manual trans in the 1970 Chev I had to fabricate my own clutch linkage.

The engine mounts on the 68 as well as my 70 Chev are very different from the later model year mounts. I had to fabricate my own engine mounts since neither the old or new would work with the frame cross member. I suspended my engine in place and then fabricated the mounts in place. The hardest part was tack welding the parts in place due to lack of clearance and lack of light. I removed the tacked parts and finished the welding.

I used the original 1970 radiator, but I had to change the radiator inlet (upper) and maybe the outlet (lower) to match up with the 6.2L hose. I never had any over heating problems using the old radiator even though it was only a two row radiator. I had to cut the 1970 fan shroud to fit the 6.2L fan. I installed an air cooled oil cooler in front of the radiator since I didn't have an oil cooler in the radiator.

When I bought our 1986 Chev. C30 DRW at Gov't auction then I swapped the 6.2L out of my 1970 with the 350 gas engine in the 1986. The 1986 has a manual and a 4.56 rear end. I just had to rework the electrial wiring which was pretty simple. And the 1986 already had a fuel return line to the fuel tank. I also replaced the 6.2L needle pilot bearing with the solid 350 bearing.