John, Chris.
Thanks very much for the information. Not quite as easy as I'd hoped but no ureasonable difficulties. $64,000 question now is whether to keep the standard disk or replace with a worm wheel and do my proposed Senior / VDH style dividng head modification using the Vertex rotary table dividing disk and accessory set I have in stock. That particular project went on hold way back when workshop enlargement let me swop my Chester Lux stye mill for a Bridgeport so I had room for a proper dividing head, Edgewick "Cincinnatti clone" in my case. Superb but mamma mia it's well heavy! A modded spindexer would be more handleable for jobs within its capability.
I considered fitting an alignment key in a rather similar manner to John but as a "need it now, right now" temporary fix simply milled the edges of the base parallel to the spindle. Like most temporary expedients of adequate performance its become permanent. For ordinary work pressing the machined side up against a parallel in the Tee slot when clamping gets things close, order of a thou + per inch taper. Precision and longer jobs need indicator alignment anyway so the key just gets in the way. As with many older machines my Tee slots aren't that wonderful so a practical key is unlikely to do much better.
Other thing that needs sorting properly is decent spindle clamp. That alloy screw on top works I suppose but doesn't inspire confidence. Proper lever operated screw up two collar clamp running in a cross wise bore underneath the spindle looks the way to go.
Clive