The problem is that with cars this old and different build methods, the tolerances are all over the place. repairs over the years, previous restoration and replacement of rusty parts all contribute to dimensional differences.
With any classic, one has to be prepared to do sone shet metal 'adjustments'. I have had Heritage panels which seemed way off, but I once bought a Mafgnum front wing which arrived at 2.50 pm and was fully welded on by 4.35 pm. It fitted perfectly, but that was just luck.
The M-Machine combined inner & outer sill sub-assembly which comes with quite a wide floor piece is excellent. You just rebate the floor to suit and 'adjust' at each end as necessary, then weld it all in. I reckon it saves around 3 to 4 hours labour.