Just to answer myself: When fitting a Zetec + mk5 Escort casing MTX75 + FRS Torsen + FRS clutch, I was able to shave some aluminum from the bellhousing at the one location only and and get the clutch pacakeg to turn freely. I tested it with no spacer plates so
I have some marginal when I use the one.

What is catching on the bellhousing, is mostly the cluth plate screws. It would also be quite easy to pocket the screw heads deeper into the clutch cover.

I also compared the mk5 Escort MTX75 and FRS MTX75 casings side to side, and it is clear that the FRS casing has a ~10mm longer lip on the bellhousing pushing the gearbox more into the left chassis leg.