I can understand why - after the modifications, and regardless of who does them, it's still a Colt. But I would have fewer regrets using a re-issue Series 70 than one that has survived 25+ years unmolested. I think the older guns are built better, but I would still feel bad for altering it.
An option might be to watch the various auction sites for an original Series 70 that has been tricked out, or otherwise used and abused, and make whatever changes you want. Then send the whole thing to Colt for a reblue - cause nobody does blue like Colt (IMNSHO).
On a full house custom most parts except the slide and frame are striped off and replaced. So, unless you have a desire to start with a base gun the is more rare and correctable, it's a no-brainer. Use the current production Colt S70. They are equal or better in every respect.
My first choice for custom build is "pre-Series 70" Colt platforms. However, the early model guns I use for custom projects are worn and/or abused to the extent they're no longer collectible. Beat up early Colts are still fairly plentiful and affordable. You just have to be patient in your search. My second choice for custom build is the reissue Series 70... which also makes an excellent base gun. Good luck with your project.
I picked a series 80. I kinda like the firing pin safety. I know, that's not popular in these parts but I just got my 22 year old series 80 redone by Wilson Combat and my trigger job is every bit as good as my series 70 is..
I have used both original Series '70 guns and new Series '70 repros in full-house custom builds. The older guns are easier to find, especially if you do not mind some wear. The choice really depends on the lines of the original gun, but I prefer the older guns because the safeties are not cast. Thus, they look better when the gun is finished. The new guns also frequently exhibit some horrible frame/slide machining and fit that needs to be worked over as part of the build. I had to send one repro back to Colt because the slide just could not be fixed.
i picked a series 80. I kinda like the firing pin safety. I know, that's not popular in these parts but i just got my 22 year old series 80 redone by wilson combat and my trigger job is every bit as good as my series 70 is..