318 vs 340 Questions

Build the 340 while enjoying the 318. Of course you won't be able to swap over the parts that interchange, unless you do it at the last minute to minimize down time. Some stuff, like the heads, will be on the small side for a 340 anyway. But a set of properly done 360 heads will work very well. Definitely don't want an OEM 318 intake & carb on it.

The driver's side motor mount is wider for a 340, but can be spaced with washers and longer bolt.

I like the 360 idea too. The main difference would be the external balance which can be solved with a B&M flexplate for an LA 360. Top ends could interchange between all 3 engines. You'll be gaining another 20 cubic engines over the 340.