it's a little more complicated. the 73 tank is not available, but the 74-9 tank is. it's a little bigger and the pick-up tube is a little different. the 74-9 sending unit is VERY hard to find, i just added a little length to my original pick up tube. the vent in the top of the tank is different too, if you have the metal vent that holds in with a twist lock ring you'll need the plastic vent for the 74-9 tank and the rubber grommet that holds it in. i got my vent from Dearborn Cla$$ics and the grommet from Green Sales. check with Green Sales first to see if they have the vent, grommet and the filler tube seal. the filler tube and the seal were the same from my 72 and the 74-9 tank. like Dan says, get a new filler tube seal, it'll be crap. i got the new straps that fit the larger tank when i ordered mine too, the originals can be made to work with longer bolts and a little bending but for $25 and no dicking around it was worth it. the cheapest place i found for the tank and straps was
www.Ineedparts.com the shipping was real cheap too, i had the thing in two days! then i found out i needed all the other goodies to make the swap.