Desmondjim, I own a hadcock 228 and it was not a drop in replacement for an rb250 I replaced. You are right about the different pivot to spindle distances for the rega (222 mm) vs the Hadcock 228 (212 mm). Even though you can move the hadcock closer to the spindle in a rega cutout because the hadcock base is smaller, the pivot will be too far away from the spindle, by at least 5 mm by my reckoning. For that reason, even though the hadcock 228 can physically fit into the rega cutout, the 228 is not a drop in replacement for the rega, IMO.
Check out the spreadsheet at www.enjoytthemusic.com, and you can see that the 228 can not be aligned using baerwald, when mounted at 217 mm pivot to stylus distance, using 23 degree offset, 228.6 mm effective lenght and resultant 10.8 mm overhang.
Actually, the 242 is closer to a drop in replacement candidate, with a spindle to pivot distance of 226 mm. If you can offset the base of the 242 in the rega cutout by 4 mm, the 242 is a perfect drop in replacement for the rb300.