I have to agree that this sounds like a ton of work that would ultimately compromise the ride by raising the center of gravity and be a pain in the ass for maintenance. As someone mentioned, after you take all that structure out of the lower frame, it might just crack and end the game.
Are you just going for the look or are you trying to ford across streams on your ped? Rock crawling?
I just can't fathom raising the motor. Automakers spend millions redesigning models top to bottom to lower their motors a centimeter.
Maybe you just want a frame brace and top tank? That could probably be accomplished with an add-on second tank that drains to the (now sealed) original tank.