Don't know what motor to swap in.

73 duster slant six. I can't decide which way to go for a motor swap. I have a 318 block I could 390 stroke, find a 360 which seems to run 600-1000 for a stock engine with questionable past, or I can buy 440 motorhome engines usually with trans and low miles for $500 all day long. Anything I buy I will be modding just for fun as time allows, and any way I go I will be needing an engine, trans, radiator, and rear end. I will require power steering and a/c and this is just my weekend driver and hopefully a drag and drive car for fun later in life. Everything in me screams big block, but wondering if it's not a practical decision?