What to do?

i'd probably fix the rearend problem first,
if it lets go the drive shaft and trans.
might go with it.
the trans. would at least need checked :sprag.
i'd check to see if your spring is being damaged,
and mark housing and tube to see it moves.
maybe weld the housing.
if you've milled the heads alot, it changes the intake
and head sealing surface angles. maybe could have an effect.
hope you can fix her and buy the heads.