Charlie,
Here are a few things I did to my BB2 that would apply to the BB3.
You may want to test fit your engine to see if you need to cut the corners off of your sponsons to prevent interference.
I built a small dry storage compartment into the starboard sponson. You may want to check on registaring requirements as some have had to undo similar mods to meet their state requirements (ie- inspectors said the loss of perminant floatation foam was an issue).
Just the essentials!!
The backing plates seen in the foam here are to give the screws more to bite into when anchoring a grass rail (decided not to install- went with plastic stap eyes to hold the camo on), line cleat, and combing.
The steel eye in the transom is for chaining the engine to the boat. I did not bolt the engine on, and had almost lost the engine on my Broadbill the first time out, so this provided much peace of mind.
If you do not go with a permant gas tank, you may want a shelf under the bow deck to increase storage or place the gas tank. I found weight from a forward mounted gas tank effected the ride of my BB2, but the shelf is still used for dekes, etc...
A dodger really adds to huntibilty, as well as helping keep you warm while hunting.
Your build is really coming along well and looking good. Keep the posts coming!
Take care,
Bill