Actually, I think the best way to use console/arcade controllers on a PC is to use a Microsoft sidewinder gamepad as the sac controller (instead of a SNES controller as you suggested). Here are a few reasons:
1. A sidewinder gamepad is very cheap nowadays. You can get a used one for like <$10 on ebay. It's no big deal to sacrifice one. It doesn't matter if the gamepad is used because you only need the circuit board.
2. You will be able to connect your controller though a normal joystick port
3. A sidewinder gamepad is basically compatable with almost every PC game, therefore whatever type of console/arcade controller you interface though the sidewinder pad will be compatable with almost all games.
4. Requires less work to build.
Sorry, I didn't mean to offend you. I'm glad you brought up the topic. I don't think that most people knew you can create your own PC controller. I just want people to know that there are other options then using a SNES pad.