Dave,
I did not modify the bunks for the Belhaven trailer. Laid flat the way they are, they deflect pretty well to match the bottom of the boat. My thinking was to have the good support beneath the 350lb keel and then hold the plywood portion of the boat up with the bunks.
One annoying aspect of bunk set-up is that the bolt holes for height adjustments are in one inch increments. I got it close but should really go back and put a spacer between the bracket and the bunk to get the forward elevation of the bunk correct. Lots of "trailer trial and error", really no other way around it.
One last recommendation specific to the Belhaven - Since she needs 10 inches to float and is pretty big to push around you really want to keep the boat as low as possible on the trailer or you won't be able to launch at shallow ramps.