Help with Rear end gear ratio

Do you have any idea how many rpm's you will plan to push the 318 ?? My son uses 3.91's in his Dart with a 400 and he has 26" tall tires. He was shifting at 6400 and trapping at 6500. But last time out he was pushing it to 6800 and ran his best so he needs more gear but its a street car. My 63 has 4.30's and I run a 29" tire. I shift about 6100 with my 440 and trap about 6300. Depending on the heads and cam you use will have alot to do with the rpm the 318 will pull. I would say with a 30" tire I would run 4.30's or even 4.56's. Ron