To properly cut the sight it needs to be slightly tapered, this is nearly impossible to do by hand, and if you go too far then welding is in order. Nothing is more irritating than a sight that constanly loosens up or has a gap in the dovetail. Your actually having to cut depth, and then width at an angle, plus maintain the dovetail. And yes, the Colt slide is hard, I would say second only to Kimber's.
I would reccomend you get in touch with Don Williams at
www.theactionworks.com and let him cut it for you. He has the equipment, and mailing just the slide back and forth is cheap. He has done nearly 50 for our students flawlessly, and I'm sure hundreds if not thousands for his customers. He also likes to help home smith's with this kind of machine work and is extremely reasonable. Let me also give a plug for Chuck Rogers,
www.rogersprecision.com , either one of these men will do perfect work, let their workload and your timeschedule decide which one to use.
Save your labors for polishing, doing the carry bevel, fitting a NM bushing, checkering, etc...