questions

[crashz]

Welcome!

While the body is off the frame, its a perfect time to freshen up the transmission and transfer case! Otherwise you'd be doing the same work from underneath. But opening up either one is not exactly for novices. If it were me, I'd take them out and bring them to a reputable transmission shop to be rebuilt. Then you can install with new mounts and such.

The engine is another matter. Do you know the condition of the 383? it might be a screamer, could be set up for a drag car or a nice truck engine, but I don't know. My personal opinion is that the 454, even stock, will give better daily driving in a lifted truck with big tires. If this engine is freshened up, it will drive very nice with good torque in a usable range. The small block may be fine as well, but it depends on the set-up. Each will use some gas in a lifted 4x4.

The 383 will bolt right up, but the torque converter may need to be changed based upon a more appropriate stall speed for the small block. The motor mounts are slightly different too, but I don't think thats an issue.

A quick way to check your gears if there is no tag on the axle or you don't trust is is to block up the front tires(so the truck won't move), lift one rear tire enought to turn it (if its got a limit-slip or locker, you'll have to lift both rear tires). Mark a spot on the tire with a refence point on the floor. Turn the pinion and/or driveshaft (truck in neutral) and count the revolutions it takes to turn the tire 360 degrees. 4 turns is probably 4.10:1, 3.75 turns is likely to be 3.73:1, 3.5 turns is likely 3.42:1, etc.