I keep about 4" minmum from the exhaust. I like to have a solid crossmember behind the tank with no protrusions going forward. Just plain ol 20 ga steel is my prefered material as it is easily repairable. Think solder the hole not weld it. The tank should be cradled not hung from side brackets. A drain is not a bad idea. Use a flush fit plug not an extended one. The fuel draw and filler must be on top of the tank. keep square corners and flat surfaces to a minimum to prevent flexing of the tank.