EFI return line, use emissions return?

It is too hot in Norway!

You could use the emissions hose. I have seen where people return thru the filler tube. It is probably too small (1/4"D) to be a proper return line, so you probably need to keep your existing bypass line to regulate pressure. The new return line would just aid cooling, by getting more fresh fuel to the pump inlet. I would be concerned if any rubber lines in the part that loops up thru the trunk. It would be pretty scary to have a fuel spray in the trunk.

Perhaps a better approach is to run a new 5/16" return line. If you are currently using the factory tube for supply, run a new 3/8" supply and use your factory tube for return. You could then do away with the bypass T. Get a new sender with 3/8" supply and 1/4" D return tube (~$45 new on ebay). I did above in my 65 Dart (lookup post). My problem is that I run 9 psi on the return side with a Walbro external pump (225 lph I think, but could be 190 lph). Most EFI systems suggest <5 psi. The main restriction is probably the 1/4" fitting on the sender. Next time I have it off, I will swage the entrance for a smoother transition. A 1/4" return tube should work if shaped more like a venturi. Not a problem now since I have a Carter 2 bbl carb on until I get around to EFI.

Excellent that you got the megasquirt working. I haven't heard of many Mopar success stories, but we are mostly a knuckle-dragging carb twiddling bunch.