Although not a drill press an inexpensive alternative if you have a hand drill or cordless drill driver is one of these:
http://www.amazon.com/Wolfcraft-4525.../dp/B000JCIMEA
I haven't drilled frame rails with one but I've gotten good results using one for other jobs. If I were to use one to drill the rails I'd build up some plywood (and whatever else to shim it to the right height) on either side of the rail for support. Hope this helps...