Best spaces for amps are either behind the seat, above the spare tire, and vertically adjacent to the spare tire "hump"

Those 3 spots will save you the most space. Above the gas tank is always an option. Its a really tight fit though. And I've never liked the thought of putting something that can get hot right above it(not to mention power cables and ground cables etc...(both very "sparkable" items). It is probably 100% perfectly fine, but just one of those things for me...

 

FWIW, gasoline has an ignition temperature of around 495 F, so unless
you amp runs really hot... and as for the sparks, there's a current
running into the tank itself (gas gauge sending unit) - I'm not too
worried about the wires outside of it.