Coleridge: I have never owned a Remington 700 Classic in 6m/m Remington so can not vouch for that offerings barrel length.
But I have several (seven) Remington 700 Classics in calibers from 17 Remington on up to 7m/m Remington Magnum and they all have 24" barrels.
I am guessing the original Remington 700 Classics in 6m/m Remington probably also had 24" barrels.
Maybe the new owner can measure his Rifle and see how long it is for us.
Hold into the wind
VarmintGuy
