Hmm, later cars are obviously a bit different to the 70s ones structurally.
In the 70s, the prop shaft cover was steel and welded to the gearbox cover. The rear part of the metal-work is only screwed to the rear woodwork so not only exists the possibility of the wood cut out being fouled, but of the metal work being to one side and the prop hitting that.
The prop hitting the wood sounds like wood being attacked, and I'd suspect if the prop was hitting the cover, a metalic sound would result.
It's all pretty basis stuff, you just need to remember the springs move sideways quite a lot at the axle so the analysis of what is hitting what ought not be too difficult. I suggest you don't hurry here, and even the act of explaining and showing the problem to another will probably help.
I've never asked a dealer about this, but they must be pretty familiar with the problem and cures.
