I have a different set of numbers for my 1984 J30, I also had the luxury of a scratch deck, newly recored, to work with.
My fairleads dictated where the bases should be and I did my best to replicate the original placements. I moved them outboard a bit but not fore or aft.
Here are my measurements. Mine is a double rail pulpit. I hope I am not confusing the matter for you.
Pic 3056 is the original layout.
Pic 3810 is new layout.
When i recieved my Pulpit from Whitewater, i was worried that it did not fit correctly as it did not sit flatly on the deck. However at their urging, i wrestled it into position and bolted it down. It IS fine even if not perfect. It is a replacement on a repaired deck and there were bound to be some imperfections.
2 years later and I never see the errors of the repairs anymore. I love this boat!