As for a 340 cam in a 318, we have all done it. Some of us have even paired it up with 360 heads, and their 318 ran great. In 1972 the advertised Comp ratio for a 318 was 8.6:1 and a 340 was 8.5:1. These numbers seem right to me, since the compression height is the same for 1972 318 and 340 pistons and the 72 J heads had slightly bigger chambers. That being said, you have to measure your own compression ratio with what you have in your hands. You can also mill the heads to gain compression and mill the intake side of the heads to keep everything in the right relationship to the intake. The 340 cam was the same as earlier 340 cams except the 1968 four speed 340 cam. Are there better cam grinds available today? Probably, but I do not go cheap on a cam or lifters. The cam is a major determinant on how the engine will perform. I used to run Isky cams and lifters, never a problem. Now, I just call Jim at Racer Brown and tell him what I am working on and what I want. He figures what to make and I buy it. Other cam grinders can do the same. Why go off the shelf, sounds like you have time to think this one out.