Linear ways: replace just the trucks, or both trucks and rails?

Greetings:
I've got a Ganesh Cyclone 32CS, which is a sliding headstock twin spindle not-quite-exactly-a-swiss lathe.
(no guide bushing, but otherwise, it acts like a Swiss.) It's not a Citizen, but it's been a very solid workhorse for years.
So the subspindle is out of alignment. 0.0025" down at the nose. Which I can comp for on 99% of our parts.
So it's not hurting anything right now.
Ganesh's local service contractor can handle it, but what they want to do is replace both of the subspindle X rails, both the rails and the trucks. Their take is that the rails and trucks are manufactured as matched sets, and you really can't just replace the trucks, or one set of anything, without replacing the whole mess.

The reason this matters: we blew a set of trucks on the subspindle X axis about 3 years back, and Ganesh's service guy just replaced the pair of blown trucks (forward), not the rails, or the rear trucks. New service guy is of the opinion that this is probably the root of the alignment issue: non-matched trucks on old rails, not matching the rear trucks either.

A sometime machine rebuilder who I respect is of the opinion that you don't need to match grind the rails and trucks, and swapping just one set of trucks, on one rail, is perfectly fine.

I can see both points of view, and am looking for second opinions? Replace the trucks again, or do the whole mess, trucks, rails and all?
I've already gone round with Ganesh about the cost of the replacement rails, so that's not an issue.
What I'm after is to understand whether or not it's really necessary to replace the rails and trucks as a unit, and then what tests to perform to certify that the thing's back into alignment after it's all done.

I want to make it clear here that while I'm not exactly thrilled with Ganesh at the moment, especially regarding what may have been a fouled up service call a few years back, the machine itself has more than earned its keep, and I'm quite happy with it. A good part of this may be because I didn't know enough at the time of the first service call to spot a potential problem.
Thus my question here: what really is necessary, and how do I inspect to make sure it's all working right when this one's done?

Regards,
Brian

If the block was damaged in a crash, bolts snapped in it, or other general life failure then you replace just it. If there is wear from general use then you replace blocks and rails.

The first set of blocks died from semi natural causes. They started leaving bits of seal all over the place, but we caught them before they started shedding rollers. The machine has a really handy "walk thru" mode that does a lot for preventing crashes.
It's never had a major crash that I know of, and I do the setup, so I'd know.

Thx,
Brian

The balls, or rollers, are sized for a block to rail set like nuts and screws to get the correct preload. With standard accuracy and no preload you can swap blocks and rails willy nilly but this is not something you will find on a metal cutting machine you would want to use. For your needs you need to get a set of rails and blocks that have the correct preload AND have been ground to a tight tolerance on stack height and if memory serves the side reference surfaces. The standard tolerance on stack height for a size 30 rail and block combo is .004" total, it sounds like this is what you are dealing with on your sub spindle. You probably want around a .0005" tolerance with the tightest preload which is more standard for lathes and mills. You could shim the low blocks to match the high ones but I would bet there is no preload on the new blocks. Don't freak out about this being a low grade fix, it's common to shim the blocks on the side to align the axis.

The seals don't keep the balls/rollers in, the end caps do. Be real careful when greasing if you do this manually as you can easily blow the plastic end caps off by pumping in too much grease, and then the balls/rollers will dump out.