Pre-war fat tire bikes are the best for this, the 24x3 tire isn't as tall as a 26" tire/wheel combo...just a little smaller, it still gives the appearance of a 26" wheel!Rebuild the rear end or at least spread the original stays. There are extra fat tires I have seen with extra fat rims that would take a custom rear end. The primo set up I think. Surley is offering a fat rim called Large Marge for the Pugsley frame and fork.
The Endomorph tire is 94mm wide (3.7") x 740mm tall (29") on our rims. It’s the highest-volume production bicycle tire on the market at this point. And, at 1260 grams, our 60 tpi tire weighs 300–400 grams less than lower-volume 3"-wide DH tires. Most 3" DH tires hover around 1600 grams.
Thankyou Tom,A little lateral misalignment will not hurt. I wouldn't go much more than 3/8". Check your sprocket too. Most of them have a 'dished' design whereby the teeth are further to one side than the other. If the teeth on the sprocket are toward the wheel you can reinstall the sprocket with them on the outboard side instead. This won't give you much, maybe 1/4" but that might be enough to give you the tire clearance you need. Make sure your wheel is centered in the frame also and not cocked to the left. You didn't say where the chain is hitting the tire but if it is the lower side, the one with the tensioner, you can get some clearance by moving the tensioner wheel out a bit. Make sure it is running true and you don't have excess chain slack. You want 1/2 to 3/4" slack in the chain measured at the center of the chain span. Hope these suggestions help you out. Good luck.