Harry - If you've the time, I (and perhaps other Creekers) would be very interested in some close-up photos of the wagon vise design. There's a lot of variations on the way the guides are handled, and I'd be interested in your solution.
I've another bench design (number 6 for me) rattling around in my head, and I'm thinking "wagon vise" after the last bench with a tail vise. After I finished it I swore I'd never make another tail vise ever again - way too much PITA.