318: .030-.050 over and Stroked to 340
Its complicated and if your not 100% into building the ULTIMATE engine, dont worry about it.
CI does matter, but when it very close, like 20ci, its not that much difference. But a 318 and 340 built the same will run just about the same.
.01 quicker in the 1/4 for a 340 over a 318? Thats hanging with a 340 in my book.