I believe 3/8" inner diameter line is pretty standard up to 500 ponies.
I forget, Shane, is your car fuel injected or carbureted. If fuel injected, the general rule of thumb is 80% of the feed line diameter for the return feed, so 5/16" inner diameter would probably work.
Also, there may be some concerns related to what kind of pumps you have and where they are in relation to your fuel filter. For instance, some electrical pumps don't like being 'pullers' so placing the fuel filter between the pump and the tank adds some resistance that might lessen the efficiency and life of the pump...