Okay I have a 1985 720 2.4 5 speed...I put a 1971 521 rear end in my truck cuz I needed a shorter rear end....Everything works great...But now my truck in 5th gear at 65 is running high high rpms...Will my gears out of my 720 rear end bolt-in the 521 rear end or do i have to find 521 gears...any help or link to 521 higher gears for sale if 720 gears wont work...