Either one would work. The 318 can get 2-3 or so better mpg than a 360 all else equal. The 360s all came with better heads and carbs. Some 318s from the '80s also had those same better heads and carbs though.
You don't have to get a V8 K member but they are easy to find since F and M bodies use the same K member. Later M bodies only came with 318s.
You'll need the trans with the motor, doesn't matter if you want to use a 904-based or 727, stick, or overdrive auto. The slant six trans won't bolt to a small block. The rear may or may not hold up well if it's stock, they did put 7-1/4s behind 318s but they didn't always do well, if you just drive it you can be okay for longer. You'll also need the right trans linkage for the carb you plan to run, 2 or 4 barrel. You should be able to get everything you need from a V8 equipped F, J, or M body car.