I worked out the arm protrudes 5/8" too far into the release bearing.
So, I drilled and tapped the bellhousing and just moved the bloody pivot!
Now it's bang-on. I mocked up the Z-bar and it actually improves clearance between that and the starter motor so it appears there's no downside
I just need to order the retaining springs and it's done. The gearbox will then hopefully be off to Duncs for a freshen-up.
No-one will believe you...