I have no idea why the P360 would send left+right simultaneously. There's no game that would ever use L+R together. He's probably wrong, because there are 4 separate triggers for the 4 directions and a common ground; to send left/right together would require the joystick shaft to be in two opposite directions at once.
EDIT: Now that I think about it, I wouldn't listen to him at all. If what he said was true, all PS/2 keyboards would have non-functioning Ctrl, Shift, Alt, Enter, etc. keys.
Anyway, if you want to hack a gamepad to use for PC (I've hacked a few myself), use this one:
http://us.thrustmaster.com/products/d_prd.php?p=T44&fam=5
It has a common ground and digital buttons, so wiring will be relatively easy. But, I don't know what kind of power is supplied, so you may not be able to give the P360 the approximate 5V it needs, if you want to use it.
Be prepared to do a LOT of reading here:
http://www.shoryuken.com/forums/forumdisplay.php?f=177
Happ produces the authentic parts used in nearly all American arcades. You can buy directly from them, though there are several smaller resellers that sell the parts for less.
http://www.happcontrols.com/
Inappropriate, you post on SRK?