Apart from the 5/8" drawbar (and not the alternate 12mm size) unless I'm wrong, an R8 is an R8 - I'd expect it to fit.

Whether it's the tool holder that's out of spec, or the machine, I'm unsure.

From previous conversation you demo'd without a toolholder installed and still had a binding spindle so clearly it's not the toolholder that's at fault with yours. I think you also demo'd without the belt from the spindle to the motor - the motor was erratic but very low speed - I guess 10rpm (default from start on a Sieg) so I wouldn't expect the motor to be super-smooth at that RPM.