Regarding the axle, can you photograph the old and the new? GM made the spring pads the same width for a very long time. I swapped the worn out T150 (banjo shaped housing, no side bearing races exist for it) for a later T170 (round housing, bearings somewhat still exist), it just bolted right in, even the U joint and brake line. The T170 is probably what's under the '76. When I did the swap back in the '90s, it was the modern option and you could still buy taller gears for it at the dealer. Now of course, it's totally deprecated however you can at least get bearings for it, and one with a taller than usual 6.50:1 ratio occasionally pops up on Vander Haag's.
At least with the T170 you get Bendix style brakes, mine even had self adjusters IIRC. Brake parts are commonly available for it, too. Went from 15x4 to 15x5, it seemed to stop a little better.
