Ya the zapper board is a bitch to make fit. I had to actually cut and file down both edges of the pcb to fit within the gun housing.
Also I believe there is a slight difference with the ground loop setup depending if you have a dual monitor or not? Don't quote me lol

If you have a dual monitor, then it should be wired like this at the plug side. I managed to pick one up from
@apswann
When you say you found another gun, is it a Nintendo gun? The reason I ask because when I obtained my lightgun it actually had a completely random pcb inside(I believe for a Konami?) and did not work even when wired correctly , which is why I ended up converting to a modded nes zapper pcb.