Sorry, having trouble posting and editing. Here's what I wanted to post before. Maybe it will work this time:
If your boat trailer is set up properly, you should have 420 lb (10%) tongue weight. This weight is part of your payload, since it goes on the truck suspension. Most extensions will reduce your hitch rating, so if you need an extension, a SuperHitch is needed. I also tow a similar weight tandem boat trailer with a good size tongue, and found a super extra long drawbar does the trick with no de-rating and no negative effect on the surge brakes.
https://www.etrailer.com/Ball-Mounts/Hidden-Hitch/80232.htmlhttps://www.etrailer.com/question-29445.html