Now you guys are making me rethink my plan.
So, this is what comes out of the pressure regulator for a vapor return line:
It is sealed with an o-ring, the threads are 1/4x28tpi. Doesn't match any fitting standard I can find. The hole on the inside is sized for a post on the diaphram. The exterior is a hose barb for 1.8" hose. So, I need to get something out of this and back to the gas tank. I can't find 1/8" hose barb adaptors anywhere. 1/4" and up are everywhere.
My thought was to use nylon reinforced fuel line from the auto parts store, held on with a hose clamp. Put firesleeve over that.
To get thru the firewall, make a steel bulkhead fitting like:
Not as fireproof as stainless, but would 1/8" mild steel hold up to a fire longer than 0.025 (or whatever the firewall is) stainless? Standard AN fittings don't come in stainless, so pretty much using regular steel everywhere else.
Run the same kind of hose back to the tank, don't need firesleeve behind the firewall. Adel clamp it to the existing fuel line for support. For the fuel tank end, make the same fitting without the interior barb.
This is how far I got today before my tools needed sharpening again:
Thinking about it again today, I could get some 416 stainless, try to make something that replaces the part coming out of the pressure regulator. Make it have the 1/4" inside with an o-ring, then have a 37 degree flare on the outside for -3 hose. Could then use standard AN fittings and 3003 tube back to the tank. Would need to get the 3003 in place inside the cockput, would be a chore.