The "self-priming" equalizing line in your bottom picture won't work. It can't self prime unless the level in one tank is a bit above the top of the line.
Unless the equalizing line is always below the bottom of the tanks. And even then, the draw will tend to empty one tank over the other unless the line length and everything else that causes restriction in the draw lines is equal (which isn't going to happen.) I think you will need a selector valve. I built two saddle tanks for a '72 Chevy PU I had and used a 3-way selector valve. Really pretty easy. Unless you forget to refill the inactive tanks and switch from a tank that runs dry to another empty tank.
But on your vent line, and easy way to deal with that is to connect a tube between the two fill pipes that's above the level of gas in the tanks.
I'm with you on the lower picture... it would probably get messed up by sloshing when the tanks get low.
Your inter-tank vent line idea is pretty interesting... a good way to equalize pressure without letting the fumes out. Perhaps also good place to put a charcoal canister to prevent pressure buildup?
I'm still hoping to get away without a selector valve assuming that the 'siphon equalizing line' is effective and am leaning towards feeding the pump from main tank only (which will be a bit lower) but i agree that having the valve (or a crossfeed pump) would be safer to have one in case the crossover siphon fails.
Many years ago we did a 52 Suburban with a Lincoln IRS which required taking a 2 gallon notch off the top rear of the factory tank. This cut it down to 16g so we mounted a 10g reserve tank on the drivers side with an electric fuel pump and a line over to the main tank. If the main tank was allowed to get to 1/4 or lower the reserve could not overflow the main tank. An IRS center section mounts rigid to the frame so the driveshaft does not move other than spinning so everything like exhaust, lines, wires etc can be very close to the driveshaft which was necessary in this install. Been working for at least 10 years.
That sounds like a really interesting build... I'd like to feel the ride in an IRS burb!
