A pressure switch really isn't needed. If you're going with the Zex kit (which I recommend and have used/owned), it uses the throttle position signal to determine WOT. Other kits like NOS and NX are adaptable to a TPS activation as well, but they come with a microswitch that gets mounted on the throttle body...it's actually a physical switch. As for not engaging before 3500 rpm, you could wait to flip the switch, wait to hit full throttle, or install what's called a window switch. You can put different "pills" into the switch to start and stop the nitrous flow at desired RPMs. A good rule of thumb is 3500 rpm start and have it stop a couple hundred RPM before fuel cutoff.

As for the B1 vs. C1, the internals aren't much different in terms of strength. The C1 has stronger rod bolts which stand up to high RPM stress better, but the B1 can easily handle 12+psi on a stock bottom end, correctly tuned of course. Just as an example, my old roommate was running shotpeened OEM LS rods and ARP rod bolts to 9500rpm daily in his LSVTEC. I've known a few people making in excess of 250 whp on a completely stock LS bottom end. The problem with blowing motors generally isn't because people make too much power, it's because they don't have the proper fuel management and tuning, and end up detonating. They fry rings, crack ringlands, etc.