need help deciding a motor

You say your 273 is out of a '64? That may pose a small problem since the '64-'65s had the intake bolts at a weird angle and limits intake choices. That can be overcome by using the heads off your 318 though. I would check out the condition of the 273 and if it needs to be rebuilt, use the 318. You can buy 2 sets of 318 pistons and rings for what 1 set of 273 pistons cost. Based on the engines you are wanting it sounds like you will be happy with 300-350 HP. If that is the case why bother with the 5.7? You can get that out of a 318 with very little effort with a single 4 barrel carb. Food for thought.