Sorry I'm late to the party. Lots of suggestions. No one seems to completely understand it all though. Do NOT use epoxy or JB Weld on your bulkhead fitting please. Yes, it's very robust. But you might want to actually deal with that fitting someday. Get a couple of Stat-O-Seal washers from Oil Filter Service Company in Portland. They do online sales. And yes, I've used these in multiple bulkhead fittings with success well over 100k multiple times.
https://www.ofsco.net/ (I used to live in Portland) You can double up with one of the Permatex fuel compatible thread sealants for pipe use. 80631 or 85420. Loctite 567 pipe thread sealant also works well.
What has not been well addressed is that a gravity feed system from the fuel cell to the main tank will equalize. The pump guys are freaking out because that method could force fuel into the main tank and overflow it. Gravity doesn't work that way unless the entire fuel cell is higher than the vent line at the cap of the main tank. Odds are that you have an equalization point that allows plenty of safety margin. The volume of the cell is moot. Once the tanks equalize they drain together, staying at the same level.
To everyone that said wait until you're at reserve to open the valve to the cell, err, no. That's how you run out of gas when you still have gas. Transfer rate is not fast enough while you are riding to do this. Ride 100 miles from full, open the fuel cell valve. The fuel will rise in the main tank until equalization point, then both tanks drain together. You're not using a garden hose to connect the fuel cell to the main tank. 1/4" is too small. 3/8" is harder to route w/o pinching it. 5/16" diameter hose works best as a compromise of size Vs flow.
Figure out how you will mount the fuel cell and where the tanks will equalize. Then how much volume in the cell is above that point. That's the amount of fuel that can transfer to the main tank before equalization. As long as you're below that amount, minus about a half gallon for fuel consumption during transfer, you'll be fine. Some experimentation will teach you how long you need to wait before opening the valve. You could likely wait until the fuel gauge drops to half and be fine with a safety margin. You'll see the gauge come back to full as fuel transfers and then it will equalize and start dropping again. When you drop back to half, you'll have half the main tank and a significant amount in the cell still.