Well change 546. After getting the floor out of there I was able to reassess my options. I pulled the FC tank from the SA and fitted it in there. It appears that I can make it work. These things happen when you're making it up as you go along.
fuel tank 1 by
Kevin Frank, on Flickr
fuel tank 3 by
Kevin Frank, on Flickr
The downside is that I will probably have to use one of the Spin Tech mufflers from the SA behind the tank as they are narrower. They were a bit loud anyway and I can use the RB Muffler in the SA when I get to putting the turbo motor in that. I could use the RB muffler in the MG, but I'm afraid there would be too little air space around it for me to feel comfortable. Especially considering how hot a rotary gets the muffler. It will also necessitate running dual 2" OD pipe to just before the muffler. It's going to be that tight of a fit.
The up side is that I will have the same or better cruising range as a stock FC and the pieces will be stock replacement stuff designed to work together. It will also save me about 1000 dollars for fuel cell, pump, fuel pressure regulator, and whatnot.