318 Mopar vs. 340 Mopar

Well, it boils down to what ever comes your way first? Then I'd wait for a 360. I did a 318 in my '74 Duster and I'm not complaining an Iota, but it is hard to ignore a 42 cube jump and most of it all is in torque.

The 340 is an expensive item to get just in the block itself.

IF you get a 318, you can get them to move well. It'll just move a whole lot better with a 360. The only thing a 360 can do that a 318 can is get better mileage (on equal parts of course)

318 pistons are a tad higher than a 360's. All else is equal really.