I put Guzzler 400 pumps in two Cohos, though I mounted them on the cockpit side of the bulkheads. One bulkhead was the 4mm plywood one that comes with the Pygmy hatch/bulkhead kit, the other was made up from 1/4" cedar strips with squared edges. Both were glassed on both sides with a single layer of 6 oz., and the fillets were glassed on both sides as well.

Both are very strong; there's no deflection in operating the pumps, and there are no funny noises like cracking or squeaking. You could go with less, I suppose, but I don't see the need to go with more. Even if you use the pumps with the supplied spring, which I contend is way over-strong, you don't put all that much force on the bulkheads.

I've asked others before, but had no responses: How do you manage to get the 1.5" plumbing stuffed in there? I went with the 400 series because I thought the 1" lines would be tough enough to get crammed in there, yet I still had to modify the fittings supplied with the pumps. Also, my 1" exit port is small enough that it's not an eye magnet; it's about half the area of a 1.5" line.