First the M-3 Lee/Grant. Almost the same running gear . A little nips and tucks hear and there burn off the cheverons off of the track pads. A full deck reconstruction. But his is DOABLE..
Second a M4A1 Sherman with a cast hull. A little

Third so you Commenwelth guys don't get left out a Sherman Firefly. This would be the easiest because mostly the turret is what changes. Longer gun counter balance that kind of stuff. If you had this you could give those Tigers what for. I think that 17 pounder is the only thing that we had on a tank that could penetrate that thick Tiger armor.
I know , I know we want the T-34. But to do a custom T-34 who would pay $500 for it ? Not too many guys a few but not too many. If I had 5 confirmed buys for the T-34 at $500 I would do it. But I don't think that that would happen.

So if there is interest this could be a good custom to do.