Rear end Swap
8 3/4 if you can find one, would be the easiest/best
If all you plan on running is 300 hp, and no slicks an 8 1/4 might suffice
If you want to get creative you can read the Ford 8.8 swap sticky
For what it's worth, I've found that the 7 1/4 will last about 3 days behind a 408 ;)