Luke,
I did wonder about the engine mounts on the 2.6 Ford engine and given your description, they do sound much more resistant to movement. So far as the origins of the Lightweight gearbox go it might well be associated with the flat engine development, although, as published in ACtion many years ago, there was a five speed transaxle designed, but never built, to go with the 2.4 litre flat six. In addition the flat six test mule G'hound, FEF 2583, was fitted with the 5 speed ZF gearbox.
I did get around to a rather shaky weighing exercise, using our bathroom scales, this morning with the predictable result as follows:
Moss 'box and bell housing with damper cradle - 122 lbs
Lightweight 'box and bell housing - 85 lbs
Discussing this again with David Sanderson the combination he was installing was the Lightweight 'box with the A.C. straight six, but the specific gearbox mounts are less certain. I had a look at the mount I have, which goes with the Lightweight box and it seems to be much stiffer than the Moss 'box mount as well as having two , not one, chassis mounting holes.
I feel the reasons for the original and then continuing presence, of the inspection cover may never be explained. Certainly in the days when the Weller Six was being developed the automobile clutch could be a weak link and an inspection/adjustment access could be explained - washing off oil and/or the introduction of Fullers Earth, balancing clutch release fingers, spotting release bearing problems and so on. Nevertheless to continue the inspection cover through a major redesign when modern clutches were reliable seems odd.