Stimpy,
You certainly could tweek the reamer and make it longer as you said but with a bolt gun you would still have the portion of the head with the rim and extraction groove added to that 1.221 and your brass would be in the area of 1.290 .
I made a widcat last year on this same idea, I shortened a 303 British sizing die to the length that it measured about .420 at the bottom then sized a 30-30 casing with the shortened die and after trimming off the excess neck I ended up with a casing of 1.290. I call it my 303 Ohio Boy, with 123 gr. Hornady spire points over 18.5 grs of WW 296 it does 2260 FPS and is real accurate.
Last hunting season between myself and a friend we took 2 whitetail bucks , 1 whitetail doe, a mule deer doe, and a antelope doe with it, all one shot kills with the average shot being about 100 yards.
If your main priority is accuracy as long as you get the chamber reamed inline with the bore, have near zero headspace, and find what bullet / powder it likes I have always ended up with a accurate wildcat and the smaller the powder charge - super efficient cartridges such as this are usually your favorites. Jedman