Interstate petrol tank

All the tanks will fit but for the Interstate bowser I think you'd need to buy a new Seat and rear mudguard (fender) and lights? To get better distance, why not fit a Long Range (LR) Tank which was designed as a swap out for the Fastback tank?

I had a LR tank a good few years ago. It is a Dominator tank with a base for a Commando. The cutouts under the rear of the tank for the two down tubes are not very accurately positioned. When the studs at the front were lined up with the centre of the mounting holes the cutouts vibrated badly against the frame. I spent plenty of time moving the tank around and finally got rid of it. They come up on ebay now and again but you need a seat without ears on. It holds an extra gallon over the stock Fastback tank. Interstate tanks come in two sizes, some early ones (fibreglass)hold about 5 gallons and the later slimmer ones in steel, slightly less. Bear in mind that 5 gallons of fuel weighs about 40lb. The long range tank looks rubbish compared with the Fastback and 3 gallons should get you 150 miles! how far do you need to go between refuels?