Make sure that you get your tongue weight correct. If you don't the trailer will pull like crap or even be dangerous. I think Boatmate puts 7% of total package weight (Boat and Trailer Combined) on the tongue. You can get it close by centering the tandems on the old axle centerline but, I'm guessing that you'll have to push the axles back a few inches to load the tongue correctly. I recommend tying the two axles together as a unit at the desired spacing with a piece of square tube "heavy tacked welded" to both of them. Put the square tube directly under the current frame rails like a spacer between the axles and frame. Align the centerline of the tandem unit to the old centerline clamp it to the frame and measure the tongue weight. If it is too light or too heavy, un-clamp, adjust, re-clamp, and measure.

Make sure that the "tandem unit" is square and centered side-to side and make sure that every time you clamp the unit to the frame you make sure the measurement from the front of the tandem unit at the port frame rail to the front of the tongue equals the the same measurement on the starboard side. Doing this makes sure that your trailer tracks straight behind you. Once you get it right, mark your new centerlines and then determine how you want to affix them.

Good Luck! if you have any design questions, I'll be happy to help.