I've got a 50s Schwinn straight bar frame and I'm thinking of converting it to mountain bike. Any ideas on the rear derailleur? Maybe a Nexus 8 speed, but I don't how the gear ratios would be and if it will allow for the angles of a front 3-piece crank. You guys think the frame would be strong enough? Any other potential problems I'm not thinking of? I would want to use this for real trail mountain biking.