I have a semi-float 14 bolt axle from a 2003 van sitting here in the garage. I'm surprised at how stout it is built. There's a ton of support ribbing on the center section. Not to mention it has disc brakes. I know they had the 14 bolt in 6 and 8 lugs versions. So, that's my recommendation 14 bolt semi float with 6 lugs. A good place to look for the disc brake rearends are the 03-current 2500 vans.
If you're not concerned about rear discs, then just about any 1/2 ton 4x4 GM truck will give you a good 6 lug donor rearend.
Your 3/4 ton truck is about 800-1000 lbs lighter than any modern 1/2 ton, so there's plenty meat left on the bone even with a 1/2 ton rearend.
As far as trannies go. A 700R4/4L60 will give you a good trans. with overdrive. A turbo 350 will also do the trick behind a 283 and be super reliable also.
