A 60's-70's Dana 44, 10 bolt non drop out or a 12 bolt will work just fine. The length of a chassis has nothing to do with it. Dana 44's were in early 60's GMC's. GMC transitioned from HO to D44 on 1/2T about then I think. You may have a slight width difference you can either live with or compensate with offset. A 12 bolt is probably the best option. Any ratio you want is available. Just measure your WMS to WMS and then measure a prospective donor. There is no way to tell the ratio for sure without pulling the cover and reading the ratio that is stamped on the ring gear edge. Not a big deal to remove before buying. You want to inspect all the gears anyway. You can by a donor for $200 or you can have one built for $900 up. I just rebuilt a D44 3.92 1960 GMC rear. $500 for parts.
"A lot of modification"? If it fits width-wise, the modification is spring perch and/or shock mounts and work out the e-brake. The drive shaft issue you probably already know about. None of that is major. Even if you find a coil spring rear with panhard bar.
I'm sure there are plenty of members to advise what fits width-wise. Cletis has given you an idea already.

Figure out ratio you want, find a standard Chevy 12Bolt on craigslist with that ratio and get'er done. Some guys use car rear ends also but they will be 5 lug.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn