Hold up there, full stop.
In general, it's always modify the tool post/nut/whatever to fit. The base castings on a machine can certainly be machined as required or needed, but the machine is the long lasting bit, and tooling is more like a consumable, you may have several different options used by you or other owners over time, so tooling is almost always fitted to the lathe by modifying the tooling. And the "nut" that comes with it is basically a threaded plate that is relatively huge and will never fit regardless of what lathe you have (that I ever heard of anyway), so it was meant to be machined to fit the lathe.
That said, I'm pretty sure you will regard choosing CXA as a huge mistake for that lathe. It's kinda what I was getting at earlier, the measurements for selection are just one parameter in the selection process. That size/type/capacity of lathe is really more suited to (IMO) an AXA most likely, and a BXA at most (note again that I would not consider a piston AX or BX unless budget situation was dire indeed). There are relative size/capacity points to be made, but if it doesn't actively interfere with function, those mostly resolve down to subjective observations and choices. However I would offer an objective point. The cost of CXA blocks is substantially higher than smaller blocks, particularly AX(A) blocks from CDCO. They also take less space to store. And if you feel you need and/or can use insert tooling and want to save a few bucks on the cheaper less sought after holders, just mill them down to 1/2" instead of 3/4", no big deal (I did that to several used on my 11" lathe). And I really don't think your lathe is going to gain any benefit from the added rigidity of a BXA, much less a CXA.
So, if it were mine, it would be AXA, and the machine what is commonly referred to as a "plinth". Just a block of steel like my spacer that gets the tool post to your idea of the ideal height to get tooling edges on center. And while you are at it, consider also making one tall enough to eliminate your compound. On small machines, eliminating the compound when not needed can make a HUGE improvement in rigidity and capability, particularly with higher load flex sensitive operations like parting. Getting a solid block of suitable dimensions may be difficult and cost a bit, but it is possible. If you can weld, nothing wrong with a fabricated then machined laminated weldment. In fact, it can make a lot of things like managing the mounting tabs and pockets for nuts much easier. You can even do it without a welder, though that would be a separate discussion better managed on another thread. You can also fabricate suitable t-slots if you like. I've seen them done completely of bolt together fabrication, and saves on expensive cutters if you don't otherwise need them.
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