Hm, so in effect your engine mounts to the cowling which is then bolted on? I've not seen that approach before. I can see how with an internal fuel hookup it would present problems if the tank wasn't mounted in the cowling as well. The external fuel line is certainly simpler--here's another sample, my LittleAxe.
Concerning the bladder pressure tank, you want to use a container tube that is slightly bigger diameter than the inflated bladder; otherwise you get "false pressure" from the sides that diminishes as the bladder deflates, resulting in a very lean run. I put a 1" dia. tube, plastic or fuelproofed cardboard, in the wing at the high point in combat wings that have no fuselage--pretty unobtrusive there. A similar tube running fore-&-aft in a fuselage would work as well, you'd just have to have exterior access to it, and fuel line outside too for pinching off. Not as elegant as an all-interior arrangement but oh-so-much more practical.