In article , brownnsharp
says...

Exactly! I know it can be done well, but was it?? Boring it parallel
AND getting it to line up in height with the original tailstock
requires more than casual attention to detail. With a big enough
milling facilities, you could mount the whole lathe bed on the mill,
zero in on the tailstock, indicate parallelness to the bed, and bore
away.

Which is typically why the headstock is fitted first, and the
tailstock is done to agree with that.

Doing the headstock bore requires that it be a) at the correct
height, b) aligned in the vertical plane with the bed axis, and
c) aligned in the horizontal plane with the bed surfaces.

Jim

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