I have a way of calculating the correct rear gear for your application given the tire size and maximum engine rpm. BUT take into consideration THIS.....the formula I use only takes into consideration maximum engine rpm with a one to one transmission output and tire size. That considered, you need to look at it this way, with a one horse power engine capable of 4,000 rpm and the tire size of 30" and a gear ratio of 4-50, you will eventually reach a maximum speed of 100 mph. It may take all day, but you will get there, relatively with a 1,000 horse power engine capable of the same maximum rpm of 4,000 you'll reach the same 100 mph with the same gear ratio of 4-50 but you'll get there before you hit the end of the driveway
So all these things being understood it is up to you to consider you're power to weight ratio, maximum horsepower and torque and how far into the upper rpm ranges you want to push your engine.
All that being said, I need your tire size and maximum engine rpm and I will assume that you are running an engine with 375 to 400 horsepower, the higher horsepower and torque, the higher you can run your gear. I will also assume at this point you are running a 26" X 9" tire and if it is a small block you're probably capable of running 7,000 rpm. Which would give you an appropiate ratio of 4-30 which should run you through the quarter at approx. 125 mph and also give you a halfway decent streetable.
Shaggy