Might be time to REALLY get back to basics
1....You have mentioned your linkage. There is no reason in the world why the linkage cannot be bolted into these cars and just work. Having said that, you might have mismatched year/ model parts, bent, broke, worn
2....Pressure plate.......Pressure plates CAN be defective, bad in that they don't release properly. If you are in a city with a local clutch rebuild shop, they will have a test setup. Might be worth a shot. You keep mentioning "over center." There is no such thing. The release fingers on a BB can go nearly down to scraping on the disc
3.....As mentioned, motor mounts / transmission mounts. But the thing is, that's what the Z bar is for. Make darn sure it is NOT deflecting.
Might be time to get in there with a camera and mirror if necessary, take some photos, and compare your linkage / lever positions with other installations.
I fought a release problem in a 70 Cuda with a scattershield towards the end of my Navy days, and finally, on teardown, it turned out that disk material had gotten wedged in between the PP an clutch cover, preventing clean release. I could shift the damn thing as fast without the clutch as I could using it.