So Many Rabbit Holes...

"Just" get a 66/67 B body 8 3/4 axle.

They can be had complete, without a sure grip for under $250.

There will be debate about moving the spring perches, but it will install as is.
Even if you pay to have them moved it should be no more than another $100 or so.

You will then have a better and more easily upgraded axle and more importantly, big bolt pattern.

A front disk conversion in the future can net you big pattern there.
You can get 85% of what you need for a disk conversion from a mid 80's 5th Ave.
(get the rear wheel cylinders from the same car so the proportioning stays the same)
This should also be under $250 from a u-pull.