I'd add a note of caution before considering modifying a Plunger Frame.
While the standard cup and cone bearings are the same for both frames, the Plunger shoulder, being part of a substantial headstock forging is considerably wider, almost matching the width of the cup periphery. There is little internal overhang of the cup (if any) so the usual method of removal with hammer and punch is well nigh impossible as there is nothing to knock against. Original cups have an internal thread to take a screw in boss type extractor which can be hammered out, withdrawing the cup. Faced with a worn later type aftermarket cup with no internal thread, or with no extractor to hand, tack weld a bar across the cup and knock on that.
The S/A frame headstock in comparison is a simple flared relatively thin walled tube. The bearing locating shoulders are narrow, so plenty of overhang on the cup to take the drift, and thus relatively straightforward to knock out. To match that on a plunger frame means removing a fair amount of material. I reckon Russ means to grind away and enlarge the centre hole of the headstock, but leave a narrower width of locating shoulder.
For low mileage leisure use on a Plunger bike I would stick with the original cup, cone and ball design.
Swarfy.