Sorry if I'm late to the party here, but this could have helped you out.
From the shmups forum, 3 pages about how many buttons a vertical cab should have for shmups:
http://shmups.system11.org/viewtopic.php?t=43016Are you still working on this? Order parts yet?
If you're serious about shmups, an American stick isn't going to cut it.
Here is a mod I made for the Sanwa JLF to make it excellent for shmups:
http://shmups.system11.org/viewtopic.php?f=6&t=41451To play shmups properly, you're going to need that or a Seimitsu.
Ok, so the above will work pretty well 4-way games, but if you're that nuts about Pac-Man, you're going to want a dedicated 4-way stick seperate from your shmup stick. You can fit them on the same panel, as a lot of 4-way games only use one stick for both players.
For a 4-way stick I'd recommend either an authentic 4-way WICO leaf switch stick, or a Seimitsu using a 4-way gate. The JLF has a 4-way option, but it's not very good. Someone ought to make a better 4-way restrictor for it...