What contour and twist for a Douglas barrel to go on a Model 70 Classic action that'll go in a McMillan supergrade stock ?

I was thinking a #4 at 23".

No.4 Douglas sporter contour is indeed the closest equivalent to what Ruger is using on theirs and they work well.

Only go to No.3 if you are going for a flyweight like my Winchester Denali .375 H&H. That is the minimum.

No.5 is for a heavyweight .375.

Looks like you are right!
I also abhor barrels shorter than 23" on any bolt action.

Oh yeah, twist: 1:12" is universal for .375's.

You can go faster if you are daring, 1:10".
Ruger uses a 1:12".
1:14" is just squeeking by and might not be adequate for long monometals.