Can I use the 1/4 inch fuel line that is already in the car for my return line?

That is what I am planing on doing on my 1979 TA. either that or the vapor can line that is a little bigger. I guess it all depends on the fuel flow of the engine

The "rule of thumb" says you should have a minimum 5/16" line for the return. As Ollie mentions, this will be mitigated by the actual fuel requirements of your engine. As long as the fuel can return to the tank without backing up in the return line, you'll be fine.....