Engine number decoder

I am going to look at a few motors this week. Is there any way I can tell what year the motor was made, what type of block it is, (ie,La, Long or short block).I want to make sure I can use them in my 66 Dart. I just want to be able to swap my 273 for a 318 or 360 without changing motor mounts and such. What do I need to look for in the numbers? Is there a decoder page anyone can tell me about? Thanks