I see your point about the frame modifying, I wouldn't mess with an aluminum frame either because they are also tempered for strength and heating them up to bend or weld would cause it to lose it's heat treatment making it very weak. from looking at the pic of the tank I saw the 2 holes in front, if it has the same 2 holes in back it could be as easy as using the U bolts, or maybe using an adapter plate to bolt to the tank and drilling 2 more holes in the plate for the U bolts.
Good job with the layback post, I took mine back off and welded in a gusset and it's a LOT firmer now, no flex.
Sounds like you're really close to finishing this one, just be sure to give it some test time before the race but you should be ok.