That pagetable article is excellent. Somewhere I've seen a good description of the transistor-level (switch-level) design of the 6502 ALU, but I can't find it. But I did find
this page (now
here) with a reverse-engineered diagram of (some of) the NMOS 6502 ALU:
(Edit: I now see there's a similar diagram of this in patent 3991307 - pdf available
here)
I haven't figured out the difference between the two bit-slices. (Edit: aha, it's the inversion of carry out in alternate bits, presumably to simplify the implementation.)
On the subject of implementations (and side-effects)
this page goes into a little detail about how BRK, RTI, JSR and RTS share the same sequence.
There's more detail in Bill Mensch's patents - see the "description of the invention" in
here for example. Surprisingly readable for a patent - even covering the design decisions.
Edit: updated image URL