RE: Tanks
well i am guessing the seem problem is an airleak. so have you tried submerging it, seal one fuel line and blow in the other so its pressurized. if you see bubbles coming out you know its a leakage problem. but deffo take the primer out, when i get back i will take a photo of how i have done mine. it also might be an idea to epoxy up the seam aswell. this will reinforce that weak point and possibly fill any gaps.
as for an airplane tank, i have often thought about doing this but even my smallest tank is huuuge. one advantage is that it has a clunk though so no matter what the orientation is, it will always be sucking fuel, just like an 8ight