OK, here goes...
The tank should be mounted behind the firewall (engine mount) on the outboard side, with vertical center of tank at center of needle valve.

Since you are using a Sure Start, you need to create some type of standoff to secure the engine to the firewall yet accommodate the choke tube...or you could remove the choke tube, but that requires shorter backplate to crankcase mounting screws. I think the choke tube is 1/4" behind the mounting plate. A couple of 1/4" square pieces of wood, harder than balsa should work. One builder uses a "U" shaped piece of 1/4" plywood.

Mount the engine with cylinder pointing inward so the tubing going to the tank is pointing to the outside of the circle. Along with the tank you should order a foot or so of small fuel tubing, although in that position you will only need a few inches.

Personally, I would use the "U" shaped plywood as the firewall and a solid triangular piece of balsa on the inboard side of the 1/4" fuselage with grain running vertically, and an upper and lower triangular piece of 1/4" balsa with grain running with the long side of the triangle (gusset) on the outboard side. This should provide enough space between the gussets to allow space for the tank.

This is merely a suggestion of what I would do to use a Sure Start. Mine actually has a Cub .049A which has a tank that fits the supplied nylon mount. BTW, mine broke twice just behind the nylon mount so I installed two popcicle sticks mounted sideways from the front to a couple of inches behind the mount. So far, so good.

George

Edit: Oops, missed part of the question...rubber bands will hold the tank on fine and will allow for adjustments.