I think you worry too much about that center stringer. I mentioned before that I don't think it serves any purpose.
I wish Don Herman would chine in on this someday.
Look at this picture. You can see the 1/2 PVC pipe that my '78 had vs the stringer. Just make sure you leave yourself enough room for your fill lines and clamps. You don't want them rubbing the bottom of the deck hatch over them.
I like Fellowships idea of the plastic fence material between the tank and the tank floor for ventilation. You can barely see the little strips between the tank and the floor here. The only problem I might have is that my strips were rubber but I got the material from a guy out at the Nuclear Plant that said it was some special stuff they used around pipe fittings and flanges that would not effect the aluminum mmaterial?
Anyway, my tank had (6) of those metal tabs. I wedged a piece of teak between those 6 tank flanges and the stringer and screwed the tank tight right into the stringer. The 135gal tank has not moved since '95.
I like stricks floor the best!
