My pick for a custom gun would be a new 70 Series repro. It's new, good quality, and not valuable in the least.
Orginal 70 Series are becoming less availiable in orginal condition and more valuable. I've seen some real dog examples of a 70 Series COLTs sell over $1000.00 already... mind blowing to me. Some of the 70 Series guns are so screwed up it would cost a fortune to fix one so it were fit well enough to be worthy of a custom gun. When you look at the cost of aquiring an orginal 70 Series and then the custom work, and the fact you are destroying one more orginal gun... I think the new COLT 70 Series repros are money ahead.
Pre 70 guns have had a premium on them since the 1970's. I personally wouldn't modify one. No reason to when COLT currently builds the perfect gun to modify...
Will