Not to be dense, but it's still not obvious to me where to insert a feeler gauge. But, I suppose one way would be to add a known shim, then measure the clearance between the carrier flange and the LSD flange, then subtract for the known thickness of the shim?
And Andy, yes, I plan to be in Pittsburgh. Going to drive the GTV in Shenley as long as they accept my registration. Hope to see you there. BTW, there was a lovely GTV-6 at VIR in Marlborough livery - very pretty!