Frankly I'm not sure I "get the point." If you are gonna go for cheap, and go for 360 factory heads, you might as well START with a 360 core and put the very same money into that as they did this 318.
The problem here is you'll have to go out a d get a 360 which is an added expense. The OP is simply going to use what he has.
What's wrong with that? IDK. But I'm sure you'll come up with a gaggle plus reasons that make sense.
It seems to be a crime doing up a 318 or the engine you all ready have. If we follow your line of thinking, bigger is better, then why in the hell didn't you suggest a 640 mountain engine. It could be argued on the same level you or anybody else puts forth.