The Mustang II used model-specific motor mounts, frame mounts, bellhousings, flywheels/flexplates, oil pans, pickup tubes, exhaust manifolds, and block plates.
The motor mounts are completely out of production, and used sets in useable condition start at nearly $200 on Ebay you'll also need the Mustang-II specific frame mounts because the 302 has to be offset slightly to fit in the II's engine bay properly.
Bellhousings are the same way, Mustang-II specific for either the RAD 4spd or the C4 automatic, other bellhousings can be fitted into the tunnel, but a sledgehammer and a strong arm are prerequisites for making that happen.
Flywheels (manual
trans.) or Flexplates (automatic
trans) will present a special challenge for you; using the '84 302 means you will have to have your flywheel or flexplate re-balanced to match your later-model engine. Ford engines built before 1982 used cranks balanced to one specification, and 1982 and newer Ford engines use another, you'll have to have a machine shop do that for you. New automatic transmission flexplates are available from NAPA, manual transmission flywheels are mostly an Ebay item.
Clutch sets for the manual transmission can be found new with some work (you'll more-than likely have to dig through paper catalogs using measurements as your guide) while Torque Converters for automatic transmission cars can be a tough find unless you go used or rebuilt.
The "Block plate" is a thin piece of metal Ford uses between their engines and transmissions, I've never really understood why it was actually necessary to design it that way, but if you don't have one you'll have problems with your starter, clutch/torque converter, etc.
The oil pan and pickup tube that are Mustang II specific are only an absolute requirement for use with the Mustang II specific bellhousings, if you use a different bellhousing, any front-sump oil pan will work (though you may have to "clearance" it a bit to fit around the steering rack and K-member... sledgehammers are a prerequesite in a Mustang II owner's toolbox if they start modifying things).
The Exhaust Manifolds are surprisingly easy to get ahold of, they show up on Ebay all the time, and prices for them are pretty good (usually around $50 or so, though shipping is almost as much because they are HEAVY!!!) of course, headers are an option too, Hooker has a set still in production (EXPENSIVE!!) as does Headman (cheaper, but they fit funny.... ask someone who has a set, they'll explain that one better.) Dynomax made a set for years, but they are long out of production, they sometimes show up on Ebay too, price on those varies based on condition (I paid a little over $100 for mine)
1977 Mustang Cobra II Help.. 
