@Eddw5493 When I fitted a concentric hydraulic slave on a previous car build I used a Saab 99 concentric slave and turned an adapter ring to mate between the gearbox bolt pattern and the base of the slave cylinder. The whole assembly took the place of the release bearing guide tube, which wasn't needed as the concentric slave provided the location for the release (throwout) bearing.
Oh, and to make sure that I didn't pop the piston out due to over-travel, this is what I did:-
Push the slave cylinder piston back to the start of its travel.
Measure the travel of the piston on the slave cylinder before it pops out of its bore.
Fit an old, worn out clutch plate and cover.
Bolt up the gearbox to the engine, and measure the gap between release bearing and the "fingers" on the clutch cover.
Repeat, using a new clutch plate and cover.
Using this info, work out what thickness the spacer needs to be between bellhousing and the mounting flange on the slave cylinder.
Also used a clutch pedal stop to make sure I didn't don't pop the piston out during use, or else it's an engine-out job!