67/68 Cougars that came with a 289 or 302 had an 8" rear axle. ALL BB cars 390,427, and 428CJ came with a 9". The 427 and 428CJ cars had 31 spline axles and N case centers.
All 69/70 Cougars came with a 9". The 351's and 390 were 28 spline, Boss 302's and 428CJ's go the 31 spline axles and N case centers.
Centers could be open, or limited slip. The Limited slip option in 67/68 was the equa-loc, except for the 68-1/2 428CJ which got the Traction-loc. All 69/70 were Traction-loc ptional except for the drag pack cars with 4.30 gear which go a detroit locker.
All the housing from 67-70 interchange. For the most part the above information applies to the Mustangs too (69/70 302's were 8") except never was a factory 427 Mustang in 1968 and 6 cyl. cars got something lesser than an 8" as well as 4 bolt lug (which changed to 5 lug in 1970)
As for putting the 460 in a '68, trim back the shock towers, and expect a tight fit to the radiator. The Crites Gen II mounts and headers is a good kit. If you look through the car build section you will find several conversion (mainly 67 - 70 Mustang) that will give you a lot of good information on the swap.