Fuel piping question

You are correct that your 3/8 fittings will be your choke point. Any line you incorporate bigger than that is wasted funds because at multiple points u are choking it back down. To what is basically a -6 (3/8). Im fine with -8. But you sure dont need ten. Thats what I run on a 650 horse...7500 rpm dedicated drag car. You fail to mention your power level, but I imagine its less.
Anyways... to get the benefits of -8. Since you already welded the sump in. You run both of those 3/8 bungs out of the sump, and Y block them into a single -8. Then you now have eliminated the choke point because you doubled your effective line size for a short stretch before the -8 union. Now pick a different fuel filter instead of a 3/8 inlet cheapo, and run -8 to your regulator. Then six to each fuel bowl. And yes, you want a return line, and yes again you want the regulator near the carb. The max distance you want it from the carb is the firewall or frame rail. Any further down stream is not where id want it.