Looking for 4th axis build advice

[marbles]

I'm going to make a 4th axis for carving large blocks of foam and maybe blocks of wood. Doing a quick calculation I see that using a 200mm ballnose end mill I should be able to carve a 800mm x 1200mm cylinder right down to its centre, which is plenty big enough.

I'm going to have to fabricate most of this from scratch or adapt existing parts and want some advice about ways to do this. Attached an outline drawing of how it might look in relation to the new build gantry router. I know I need to be able to raise and lower the central axial line ie the head and tail stock centre line for different sizes of material. The head stock could be fixed and the tail stock brought in out out to meet the material. I also want to be able to unbolt the both stocks when not using them as they will restrict use of the router bed.

Head and tail stocks:
I thought it might be a good idea to repurpose or adapt existing stocks from a metal or wood working lathe. Or perhaps there is something better to use, harmonic drive. I'm all ears as I know very little about lathes etc. We also have a machine shop across the corridor at work and can get parts made up if need be. I suppose it would be good to have interchangeble heads that fit into the head stock to take different types of chuck and rotary plates.

Motor size and gearing:
I was planning on using an 8.5nm nema 34 and 100vdc digital driver but not sure what sort of gearing to use. HTD belt and pulley is a tech I know well and 4:1 ratio seems common but may not be appropriate for this particular purpose.

Chuck and rotary plate:
wondering about what type is best to hold materials. If its a block of polystyrene best if that glues onto ply ply screwed to plates, plate attached to chuck or on its own morse taper spindle or whatever would be best to take as many different fitting as possible.

Any pointers, links or questions welcome