In my experience, what brings out the big bucks is these tractors having the original 2 valve GM engine. These old engines are getting a little hard to find and really make the complete restoration.
The 4 valve DD engines are plentiful and do make some more power. Most true Oliver collectors reduce the tractors authenticity when they see the DD badge cast on the block rather then the original GM badge.
After hearing this I am glad I didn't toss my old engine when restoring my 990. I really think I spent a lot of unnecessary extra cash and time rebuilding a 4 valve DD. I only use my 990 for pulling so the repower was needed.
Very nice tractors by the way.