I just did spacered inwales on my wherry, it has drop in rowing units, so no oarlocks. I found that trying to exactly calculate blocks/spaces on a long, curved inwale so the blocks landed over the bulkheads was futile. I cut the blocks all at 2" and first  installed them over the bulkheads, then figured out the spacing. Some spaces are 2 1/2, some are as much as 3 1/4, at the end of the day, it looks great and no one will ever notice the  3/4" differential you did to make it work. My guess is that with the oarlocks, you want a full block underneath them, so plan accordingly. As long as you plan to not end up with some little 1" spaces between the blocks at the bow or stern it will look great.