I would run the loop on the vent line, to prevent any fuel coming out on downlines and in teh unlikely event o f a nose over. I wouldn't install a check, while it would prevent fuel from coming out, and wouldnt' be a trouble for the carb pump to pull open, it would be a PITA during refueling ops. Won't let the air out!.

Do try to separate the 2 battery packs. Best practice is for the entire ignition system to be in front of the firewall, separated as much as possible from any radio gear. 6-8" is good, more is better. Of course, moving that ignition pack could introduce cg issues that might require more moving of stuff, but if it reduces the possibility of RFI, it's worth it.

You do pretty nice work! Be sure to range test with and without the engine running. Engine running should not decrease the range. If it does, look to reduce RFI. Many threads on the gas engine forum to help you do this.

Blue skies!

J