Why "upgrade" a design that has worked well for over 75 years? The myth that tapered roller bearings are somehow better than ball bearings doesn't have much proof behind the fantasy. If you simply must make a change just for the sake of following the crowd, find one of the progressive size charts that all bearing manufacturers publish and do some measuring of the original bearings. If you can find an outer bearing race("cup")that will fit the bore of the hub, a good machinist can make an adapter sleeve to make the "cone" fit the spindle. You'll also need to find a grease seal that can be fitted to the hub, and have a sleeve made for the seal lip to ride on, since the original equipment ball bearing has the seal surface built into it.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!