Thanks for all the advice so far. I find the flip-flop hub mentioned in the Sheldon Brown website interesting. Still, this is just a beater bike, and I'm not going to pay $$$ for one - I'm working on getting the money for my real bike!
I'm still working on the crankset (it's cold and dark out now - a bit harder to work!). I think I'll keep it relatively simple - I'll try to eliminate the unwanted chainrings and just leave the full cassette on the rear wheel. I don't have the proper tools to take apart a splined cassette (unless I can use common-found tools?). One chain ring in the front (in theory) and seven at the back - that's my plan.
I'm looking forward to the final product.
Thanks for the help so far!