EFI return line, use emissions return?

Your pump situation is not good. The mechanical pump is already pumping against a vacuum, and not having a return is really not good.

I'd get a low pressure electric, mounted right near the tank. That way, there is very little "under vacuum" before a pump. The main line from the tank to the (now) mechanical pump sees some heat as it comes up past the engine, AND it is now under a slight vacuum, which promotes vapor/ boiling.