Here's one Drill Press method...from experience.
Tools:
Drill Press - (A bench-size model like the small Delta brand will work (not as sturdy), but I'd rather use a big floor drill press)
Vise - a cross-sliding machinists vise. Adjust the gibs (the friction plates that are tightened by three or so screws on each side of the slides)to take out as much slop as you can - you probably won't get it all.
The cutting tool - a slitting saw with arbor, endmill, etc. (the more teeth/flutes you have the better the cut...IMHO). If you buy low-end, get one for each side.
Figure out some method to support the frame while it is in the vise with the pressure of the cutter against it.
You can lay it flat on it's side, or stand it up depending on the cutting method you use.
I found the table on a small drill press to be weak. The vise is fairly heavy which would make the table deflect downward. You might need to prop it up level.
TAKE SMALL CUTS!!!!!!!
Don't be in a rush to do this job. Cut, stop & measure often. Don't try to cut to finish specs like this. You can finish up with a file.
I bought Brownell's slide-rail file for twenty-something bucks and was glad to have it... until I figured out that I could grind a small bastard file the same way for cheaper.
Make sure your setup is square to all axis.
Your cuts should be the correct depth and distance from top plane, from one end of the frame to the other. It's not hard to be cutting down hill, or shallow at one end and deep the rest of the way.
Before cutting the rails, you need to decide what point on the frame you are going to measure off of. If it's the top plane, level it off carefully first.
Get a copy of Kuhnhausen's books on the 1911. Someone may think they are pricey for just one frame, but you'll need the information to do things right. He has a video on some aspects of working on a 1911 also.
Buy good parts. I wasn't thrilled with the quality of the DPMS parts kit I purchased (it may have been a freak thing?). Check around on some handguns boards for more parts info.
Gotta go, but I think I covered it all.