My quick $0.02: I wouldn't do anything with a reed switch. The one's I'm familiar with wouldn't handle the banging around, dust, etc. If there are any electronics involved at all, use some sort of hall effect switch (that's the technology that replaced the mechanical distributor rotor), you can get these at most any electronics surplus store. Here's Honeywell's info on
solid state sensors (disclaimer - years ago I used to work for Micro Switch).
If you use a potentiometer, get a sealed one and not a Radio Shack cheapie. And of course, for any of the sensors, make sure that in a failure mode, they don't interfere with throttle or brake position. Last thing you need is something rolling around your pedals.
I don't have the electronics knowledge to be of circuit help (though it sounds like others are) but I can help with sensor selection, mechanical configuration and packaging.
Don