OK I didn't fancy buzzing out all those backboard lines to prove if the slot addressing in the 6 slot was different from the 10 slot as the experts had told me, so I went ahead and tried it anyway. There are two input output cards and a channel card to drive the additional axis:
MC301 has 64 Digit Inputs and 45 Digital Outputs
MC323 has Analogue i/o (not used in this config, and 64 Digital Input and 64 Digital Outputs
MC652 drives an additional daisy chain interface to the V axis (opposing spindle)
Now I can monitor inputs on built in displays on the controller, but I've not yet found how to drive outputs, but my reasoning goes that for these inputs and outputs to feature at the correct addresses so long as inputs on a card are proved to be in the right place, the outputs should also be ok address wise. Now there is a logistical issue testing this - the controller is on the front, and all the wiring is round the back! So I made up a long test lead consisting simply a single pole switch on one end of a wire pair, and a pair of 'bootlace ferrules' on the other end. A '1' is +24V on this system, so one wire was fixed to +24v, and I sequentially removed a selection of inputs from the termination board substituting my other wire. This way I could watch the interface monitor screen, and toggle the switch and watch bits being turned off or on. All inputs showed up in the right places so I concluded that the MC301 & MC323 backboard addressing must be what the system expected. Now for the MC652 card: Each drive (Axis and Servo) talks along its daisy chained bus and gives its status to the system the same way. Each has a rotary pcb mounted hexadecimal switch to set its 'drive number' on the bus hex 0-E are potential drive numbers and 'F' is unequipped - ie servo amp not used. Sure enough making adjustment to the drive numbers showed up on the appropriate screen of the controller so again I conclude its address as far as the system program is concerned work just as it did before in the 6 slot backboard. So I MIGHT be on a winner.

Fortunately all the i/o wires are screwless 'Wago' types that you open and close pushing a fine screwdriver in a hole, so connecting and disconnecting wasn't as hard as it might have been.
Talking of winners, I grabbed some 96 way din connector extender cards on ebay which arrived today. The system has single connector (half height) boards and double connector (full height) boards. In the photos that follow we have:
A/ Single connector extender - can be used unmodified
B/ Dual connector extender - needs slicing up and the connector spacing adjusting
C/ Tripple connector extender comprising three single bolted together with spacers - I may use two of the element of this rather than slice up B/
D/ a shot showing how close to the required spacing the dual extender is
E/ a shot of the now empty and forlorn 6 slot controller