Be careful grinding the shims down!
Is the Triumph range of shims not 0.090" at the minimum thickness?
I think the Stag and Hillman Imp both use a similar system with bucket cam followers over shims.
The reason I suggest being careful is that I have an Imp, which at one stage required a shim ground thinner than the standard range to get the correct clearance. I checked the clearance with and without the thin shim, and all seemed well, the clearance was correct.
A couple of thousand miles later, a valve dropped, hit a piston, bent, got driven back into the valve guide, which
****. The cam then tried to open the valve again, but instead, the cam follower shattered,
**** the cam carrier, and bent the cam.
What must have happened, is that the valve collets settled slightly in use, and the valve spring retainer had moved away from the head a tiny little bit.
When the cam lobe was pointing away from the valve, the clearance was correct - but when the cam opened the valve, the cam follower was bearing on the valve retainer (on the lip of the ring that retains the shim), so that the shim slightly unloaded.
That meant that the valve collets had a cyclic load on them, and the valve stem failed due to fatigue at the collet groove.
I'd have got away with it if I'd taken a couple of thou off the top of the valve retainer maybe (there were witness marks inside the cam follower), and never learned this lesson!
Anyway, just be a bit careful that using a thin shim doesn't cost you a whole lot in the long run.