I beleive DADS53 means "bellhousing". The bellhousing pattern is the same as all other 216/235/261 engines, the 6 didn't change to the V8 style bellhousing until the block change in '63. I've never heard of a US spec truck with a 235 after '62, but I have heard of the 261 being used in the 4x4s in '63. I do know that the 235 and 261 were used up to around '76 in South American spec Chevrolets.
If the truck you're looking at is a 3/4 ton, it could be a facory 261. That was an option on 3/4 and 1 tons that year.
