Don't

yet, but this is definitely a project where you need to define in advance what it is that you are trying to do.
You will have the same problems with *ANY* JAMMA-based switching mechanism -- guns (neither optical nor positional) aren't included in the JAMMA spec, so there aren't managed by the switchers. Clay's (you mention Craig, so I'm not sure that we're talking about the same guy/kit) kit does have provisions for "auxiliary" controls, but I don't think they will work nicely. Add to that the complication that not all arcade guns and gun PCBs are compatible and you end up with a very tricky situation. The situation can be managed and there are, at least theoretically, solutions to the problems, but everything depends on what games you want to play.
EDIT: Craig beat me to it ... For me, the biggest issue is that I would want to play classic games like Cheyenne, Crossbow, Hogan's Alley, Duck Hunt, etc. where the original games aren't JAMMA, the monitors are very different and the guns aren't cross-compatible.
Blech ...
For me, the most interesting opportunity is the Nanotech optical board, but I'm still on the fence in that regard (have enough other projects) and tend to disagree with most of everything that people here have been saying about it. I haven't yet bothered to research the issues that MAME has with the gun games that I want to play, so can't comment there yet.