There are people doing it....
The real diff between a 318 and a 340?
Cubic inches........ and a motor mount
A few more IQ points out into a 318 will make it run harder than a 340 with a few less IQ points put into it............
340 heads can be put on a 318......
Most here will say how a 340 will make so much more power, blah blah, but were talking 22 inches, not say 122 inches like a 440. I'd build a old 318 first, anyday. But thats just me.