Finally some Dany's Mag swap progress, and fuel line routing questions?

Personally, I'd get a return line plumbed in first if you don't already have one. Then use a 3 nipple fuel filter after the pump, that way you are constantly circulating fresh, cool fuel through the line and it doesn't have a chance to heat up and start vaporizing. You may not end up needing the 3/8 line. But if you really want the 3/8" feed, I'd still use the original 5/16" line for a return.