Modern 360 question

This may sound like a stupid question to some of you pros, but I am just starting out and learning the ropes so please don't get offended by my ignorance.

Last year a I read an article on putting modern magnum heads on an LA 318. I thought it over and purchased some with the intent of rebuilding my 318. I have since decided to have fun driving my 318 while I patiently rebuild a 360 as a replacement. My question is this. If the magnum heads will bolt onto my old block (or a 360 for that matter), would I run into problems or complications if I decided to find a complete modern 360 to rebuild. Also, (this is probably the stupidest question of all) what is the real difference between a long block and short block? Are they really that different? I am assuming that my 71 Dart has a short block. Am I right? Also will the modern 360s link up with the 904 or would I be looking at replacing the tranny as well? Thanks for any knowledge you are willing to lend.