Where to buy rebuilt engine?

Hi, I've got a 1964 Dodge Dart GT (273ci) and the engine block is cracked. I've been told finding a 273 will be very hard, and that a 318LA will fit just fine, so I should buy a 318LA instead. What are some good companies/websites to purchase a rebuilt engine from? And do you agree that a 318LA is a viable option? Thanks in advance!
Depending on which year 318 you get, Yes, it's a fairly easy swap. If you decide to keep it a 273 and want to buy a used block from that guy in Temecula, let me know. We can maybe ride together to save an extra trip and some gas. I could use a good standard 273 crank if he has one, or, if you decide to not use yours. PM me if interested.