the V8 mounts are mustang II specific, you'll need the frame mounts too (the things the motor mounts bolt to). if i remember right, one side of the motor mount is available from most auto parts stores, but the other side isn't (which is completely retarded).
your best bet is to check ebay, or some of the larger mustang II enthusiast sites classifieds section. if all else fails you can probably get them from Phil's in Alabama, his info is in the sticky at the top of the II section on this site - but he'll probably want a premium for them.
you'll need a mustang II bellhousing and flywheel because they are smaller in diameter than the ones found in other cars. since your going to use a later model 5.0 that has a 50 oz balancer and flywheel you will need to get the mustang II flywheel rebalanced from 28 oz to 50 oz.
another thing you will need is a oil pan and pickup tube from a 75-78 mustang II V8 car, nothing else will fit. and lastly, exhaust manifolds from a mustang II or you can get headers from hooker or hedman. if you go with headers, i strongly recommend staying away from the hedmans because their primary tube routing is completely retarded and you WILL smash the front tube on the passenger side the first time you hit a large bump.
good luck
oh, and if you dont want the car to ride like crap you'll nee to pick up some new springs for the front. PST makes them and they aren't that expensive.