The 3/8 will be sufficient, the 1/2 will be plenty. Your system is as small as the smallest area, so you could have a tight bend in the 3/8 line, and it will be as small at 5/16 anyway. At least, if you have a tight bend in the 1/2", it should still be as good as the 3/8... If you get what I mean?
Check your fittings too, I have some -6 fittings, that have a tiny hole in them, for the flow.... The fitting is a restriction.
Getting a sump put on your tank will cost $150 - $200, so no big deal. Any fitting size you want, front or rear of the tank.
The old Holley blues I remember, only had 1 outlet? But newer, bigger pumps have a return from the pump, back to the tank, perhaps that is one of the outlets you are looking at?
The regs have 2 outlets, so you can either just use 1 line to the fuel log, or use the reg in the middle of the 2 bowls, and use the reg as the t piece, feeding both bowls.