I'm a bit confused. If you don't need or have a return line, then run it from the main feed off of the pump (electrical or mechanical) to the brass inlet at the carb. If your running a three line fuel pump at the block, just return the smaller line for return fuel back to the tank. Then keep the feed line to the carb as normal. One suggestion I like to do, get a banjo fitting for the carb inlet. It gives you options for routing and looks to dress it up with your choice of pretty fittings and lines, etc.