DON - It's been FORTY-EIGHT years since I was around an Original! And it was only two yrs old when it was traded for the 70. Not near enough time for the frt wheel bushings to wear.
I suspect your machine shop will use 360 free machining brass. It's common, machines like soft butter... or a "Rotten Banana" as one shop owner told me 28 yrs ago. Plus it's common enough to be "cheap", nothing Special about it.
An Aluminum bronze would be almost as hard as the steel spindle, but much more expensive in small quantities.
The bearings were used to reduce friction, last longer, and since they're such a common bearing, probably cost less over-all when using the standard wheel with the flanged bearing.
I'd really stay "stock". I think the first pair of bearings I bought 30 years ago were about $3/each, last four were over $16/each. I know the first bearings were made in the US, but the last ones were not.
To make the bushings work right, you'd have to use round bar stock larger than the bore of the frt wheels, there has to be a flange on the ends of the bushings, then turn down the length of the bushing, then cut the inner length so they almost butt up against each other. Then you'd have to grease them every 10-20 hrs of running depending on conditions... plus they'd make a greasy mess of your frt wheels.