If it's more fuel.Guys have used Harley dresser tanks.I usually use Sportster tanks.I go bobber look though.They are usually pretty easy to adapt because of the weird frames.I think it looks ok with a fairly stock bike too.I used a Mustang tank on my '86.But that requires frame mods.

Here is the first '87 I did.Kinda pre-bobber.It was fairly easy to mount.I welded on a connector nut(a nut about 2" long) to the backbone of the frame and a bolt goes through the front tank mounts into the nut.The rear mount I cut the stock tank rear mount off and welded it to the Sporty tank.The downfall is you have to move your petcock or lose some fuel usage.

Honda ATC (3 wheeler) is almost a drop on, but metal tanks are getting hard to find.Small GS, CB, XS (750/850 nearly drops on), and KZ tanks. SOME Sportster tanks almost fit, some are way too long. Ditto Big Twin tanks, 3.5 gallon EL/FL are pretty close, but you'll need a frame backbone conversion kit (V-twin Manufacturing).

The hard part is the square backbone tubing. If the bike had normal (round) old school traditional tubing it would be a lot easier.

Paughco makes a killer Indian chopper tank, but it won't work on our frames w/o a conversion to round tubing and at least a 6" backbone stretch. Ditto their 4.5 gallon Mustang tank for FXR frames.  

I've tried way too many tanks on mine. I run an unknown make dirt track steel tank. Takes a 250 Rebel petcock.

Not sure what look you are after....or if it is more fuel.

The Suzuki GT380/550 and some GS tanks can work - but the width of the frame loops at the back and the decompression solenoid challenge the width of the tunnel on most tanks.

I have a Suzukii GT550 tank and the tunnel width seems like it will work with the solenoid.  I also have a Honda CB360 tank that has potential - but will not work with the solenoid.  Both tanks will require new mounts to be made.

Welding a motorcycle tank is not a big explosion risk if it has aired out for a while.......most tanks that you buy have been laying around without fuel in them long enough that they are safe.....if you can't smell fuel in them there is probably nothing to explode.  If you are in doubt rinse it out with some acetone and remove the petcock and cap and set it in the sun for a couple of days and let it air out. You can also run an aquarium pump hose in and air the thing out for a few days.

I'm looking at more of a rounded rather than triangle (stock) tank and also trying to make the lines of the bike more straight across. Kind of cafe lite if you know what I mean.
Think I need to look at my savage a bit more as even when I took my tank off last month to swap out the petcock, I didn't even notice any of this square frame/decompression malarkey. My excuse: it was cold and I was in a hurry  

What's on there at moment WD?

I've got a couple extra tanks from my current CB500 project, and I've thought more than once to see how the savage would look with one.  The fit would probably be fairly close with minimal modification, and it'd be nice to have a larger tank.  Stock honda petcocks have 2 nipples, but that's an easy fix with a vacuum plug and a clamp.

Next time I have things apart, I can mock one up and take some pics if you're interested.

Had another look at the bike today and yep I noticed the backbone is square    - why is that, what's the thinking?
Anyway I'm gonna try an xs750 tank and and maybe a CB350.
Do the larger CC tanks generally have larger tunnels than the smaller ones? ie. would CB650 be a better bet than CB250?
Also can I just bash out the tunnel on a tank to make it fit or am I kidding myself?