Guys,
I started shooting the 45 with a Colt series 70 Goldcup in the early 80's. Out of the box, the Goldcup shot 5.5" at 25 yards off bags.
I was fortunate enough to meet Russ Carniak at a gun show way back then, and through his feedback and advice from his years as a military armorer, learned how to match-accurize my own guns. That first Goldcup took 100 hours, what with all the mistakes and over-trimmed sears and hammer hooks and other parts I went through in the learning process, but it paid off. The first group of 5 rounds of Hornady 200 grain jacketed SWC's over 4.2 grains of Bullseye measured 0.37" C-T-C at 25 yards. This was using sandbags and an Aimpoint sight. I got so excited when I walked up to the target and saw this single ragged hole, I stopped paying attention and actually shot a staple into my own thumb putting the next target center up. Always stop and take a deep breath. Celebrate later.
George Nonte pointed out years ago that, accuracy-wise, you have to divide the job into mechanical accuracy (what a machine rest would do) and practical accuracy (sights, trigger job, or other things that help the shooter's gun handling). Here's are my observations of the relative importance of different accurizing steps on group size:
Mechanical Accuracy
Solid barrel lock-up in slide: 60% (This does the most for barrel/sight relationship remaining consistent).
Tight fitting bushing: 10%
Tight slide/frame fit: 10%
Match barrel: 20%
Practical Accuracy
Good Trigger: 45%
Good Sights: 45%
Grip related parts: 10%
I've been to school at Gunsite and found a lowered thumb safety helps in that environment. I also went back from a target shooter's straight mainspring housing to the original arched mainspring housing in that gun. I found that helped funnel my hand into the right grip in making the presentation (draw). What's on my list above applies to bullseye target shooting, which is what I first learned accuracy work for.
The group gripper can help if your recoil spring is stiff enough. For wadcutters we tend to want a weaker recoil spring for function, but those, in turn, often don't bear firmly enough on the group gripper to lock the barrel all the way up.
As to accurizing the old Colt, I wouldn't do it. The old GI slides were softer. They tended to batter loose and need constant armorer attention in target shooting. In particularly, the slide stop notch tends to get peened out with a lot of use, and the metal around it can crack. The modern slides are spot hardened there.
So, I would look the old war horse up in the Blue Book, and if I could get 80% of that value from a collector I would take it and put it toward something more modern. The Kimbers give the best accuracy out of the box, being made to far tighter tolerances than the Colt's, but you will pay for that. Also, to my finger the Kimber triggers are too soft and need a bit of work to really break cleanly. Colt's can be accurized, as I did mine, but the series '80 firing pin block safety makes it more difficult to get a good trigger. I designed a special jig that lets me do that, but it' not a beginner's task. The Springfield armory GI gun is what I started with for the gun I took to Gunsite. I would do it again.
If you are going to send the gun to Clark or any other accuracy shop, get a used one with a beat-up finish and let the shop re-finish it as part of the package. That will save money and, short of actual damage, new is irrelelvant on a gun that is going to have its rails hammer peened and maybe its slide bent and lapped. Look for forged rather than a cast frame if you are going to have it accurized. The exception would be a factory precision machined part, like a Caspian Arms match slide and frame.
Good luck with the project. As my own early experience tells, the potential accuracy of the 1911 design is terrific.
Nick