calboy147, your special tank holds about 2.9 gallons, .9 is the reserve. The 74 to 79 standards hold about 3.9 gallons with the .9 reserve. The standard tanks fit right on the specials.
The only problem might be the petcocks. On the early tasnks they used two non vaccum petcocks, one each side. Hook them together with a tee and ffrom the tee to your carbs.
I think the 78 and 79 used two vaccum petcocks. Not eactly sure on that. If so just use a tee to hook both petcocks to your carbs.
The xs750, xs850 have larger tanks and mount a bit different. The rubber pucks at the front of the tank are on the frame, not the tank. Just need to cut away the stock front mounts and bolt the rubber pucks to the frame. At the rear you will need to alter the seat and build the rear tank mount. The tank is longer.
I think they are about 4.5 gallon. Still not a 250 mile range. You will need a 5 gallon tank to get 250 miles. And that will be from full to bone dry.
In the after market they sell big tanks for Harleys. 6 gallon I think. That would give you the 250 miles and a bit of reserve.
May I ask why you want a 250 mile range? If it is to extend gas stops while traveling then I think the Standard tank will be fine. Just do gas stops at 125 to 150 miles. Thats a good amount of miles. I shoot for around 100-125 myself. You need a rest break about then anyway. Drain the kidneys, stretch your legs.
Leo