Did ^that^ on the phone, so just changed a couple of bits;
Either way it'll work but depends how small (fussy) you wish to be with originality. ..
and "fixed gear to "constant-mesh"...
Spares books say the Plungers were made to '57, but I'm no expert on this, and am basing my comments on my own observation/misadventure
, like I don't remember noticing that difference-- even though it needed helicoils in those lugs...good spotting
Might be worth considering any measurement difference between the bushings/overall cluster length on the layshaft with regard to the milling around the bushing area, and thrust 'collar' thickness of the bush, might be a bit more end float?