Combination of bullets seated a little too far out and the differing feed-lip design.
I took two Chip McCormick magazines, one Kimber, and two ACT. Added the remaining box of slightly-too-long cartridges and a new box of proper-length stuff, threw in the range and settled this.
The CM and Kimber fed from either box with no problem. The ACT fed the proper-length without a hitch, but combine them with the overlength stuff and got the occasional FTF. So it wasn't anything to do with the new frame.
So that's settled. Now, having found a good extended thumb safety yesterday, I shall proceed to fit it(sweating the whole time to make sure I don't screw it up).