I'm not familiar with those cars but you might also check the firewall where the m/c mounts. If it is stressing or flexing a little you would be loosing stroke. I have seen it on other makes.
Did you have the flywheel resurfaced? Did you use a crankshaft shim? Since the days of adjustable clutch linkage are long gone, that stuff becomes pretty critical. If it was assembled on the ragged edge of being out of specs, then a small amount of wear would put it over the edge so to speak.