I only owned 4 steel bikes from the 70-80's. A half decent Miyata, a super nice Dawes Super Galaxy, an Italian Masi Gran Criterium, and Peter Mooney (more of a century or touring geometry). I am pretty sure they all could fit 32mm wide tires in 700c. I know for sure that anyone of those would be easier to do a 400k on than a mountain bike. The Miyata dropouts destructed one too many times touring and one of the tubes at the BB shell broke on the Dawes. The 531 tubed Dawes would probably have been my all-time favorite long distance frame. It was springy and fit me like a glove.
If OP is riding the Mtb on brevets due to costs, sometimes these old bikes can be bought for a song and if they have good wheels, they can be made into wonderful long distance bikes.