Do you actually have a real reason for wanting to change the rear end?
It seems that your concern is that it's harder to maintain your brakes, but this can be overcome without tossing out your tapered axles. It's a bit of work, but it can make things better in the future without the complications of changing the rear end.
First you have to grind off the greater part of your wheel lugs, then drill into them so they will collapse when you hammer the remaining bit through the flange, you do need to avoid damaging the holes in the flange. Drums, new ones from NAPA, are not expensive, get some new standard lugs out of a pair of later model axles. 'Later model' might include those out of a whole bunch of vehicles, like Dakotas, other pickups or post-'65 sedans. If you want to retain left hand threads you need to get these from pre-74 models IIRC.
The next point I'd like to raise is that I believe (though I don't remember where I got the information, probably John Kunkel on Moparchat) the '64 rear end is wider drum to drum than other B-body axles prior to 1970.
And then there's the issue of your centre's spacer block being a different size, so if you went to put your pumpkin into a later housing you will run into trouble adjusting the axle clearances.