[Mr70]

A Black LS-6 similarly equipped but with a TH400 just sold for 80K a few weeks ago.Down from it's higher asking price,but fully restored w/Buildsheet,docs & photo history.
I know of a M-22 similarly equipped restored twin to yours that can be bought for 95K,with no takers right this very moment.

If you are looking for possible pedigree,besides the buildsheet,I'd aim for that rear axle tube suffix if the rear looks authentic to you and was never out.That Top hose,shroud,cover & tank say 4:10 to me,which spells solid liftered engine only.
I would also do a past owner search,especially if it's from your neck of the woods as it would be most convenient for you then for anyone else farther away.
If none of what I just mentioned pans out,I really don't think you will be as upside down on the car as you think when done,judging by the quality of work you've done in the past.