Re: reverse mounting gas tank
The ones that are inside the rear rack look good, as do the ones behind the seat.
Depending on what bike you've got, you could mount that peanut tank on the factory rack. I did temporarily, and liked it enough to keep it.
Almost anything will work, Briggs and Stratton make metal cans that look possible for our application. Anythings' better than that kit tank on the top bar.
It's cooler to not fall off your bike.