Coming from a good bit of machineshop background, I totally agree with those that recommend piloting off the bore and reaming from the breach end. Also, be sure the reamer runs true on the extension.
It's not brain surgery but requires some understanding of how a reamer works and mechanical aptitude.
Tooling could cost upward of $100-150 for all the pieces for an extension, reamer, pilot bushings, etc. If you need a carbide reamer for cutting chrome bores, it'll be more.
There's a guy named Bob Day in Pico Rivera, Calif that has a biz called R&D Custom Barrel Service, who specializes in this work.
http://www.americhoke.com/ Jim Legg used him and was pleased.