You are trying to put a 304 in a 6 cyl HornetAMX, Right?
If you're working with an Automatic, I think your existing one should bolt up to the V8, all you need is the flexplate that came with the 304 and the V8 Kickdown linkage.
When it comes to Motor mounts, get a Hornet/Gremlin V8 Crossmember, and be sure to get the mount brackets (that bolt to the engine) that go with it. There are Tall brackets and Short brackets, Hornets and Gremlins (as well as Concords and most likely Spirits too) use one style of bracket while most other AMC cars use the other.
If you can't find a V8 Crossmember, I believe Bulltear makes a conversion Motor mount setup.
Your AMC 15 Rear end will need to be upgraded to an AMC 20, The AMC 20 Rear end will more than handle tons of power and abuse (yes,even with stock 2 piece axles), the lighter duty AMC 15 will usually not (I went through 3 of them with a 232 way,way back during my Sophomore year in High School ).