Welcome to iRV2.
I can't help with your weight and roof material questions , but I have had to flip axles on a fiver.
My advice there is ; because you are adding leverage to the springs and mountings everything has to be in perfect condition . When you flip the axles to bolt them on below the springs , using the original axle saddles , you must also remove the backing plates and re-install them to maintain proper brake operation. Tires, brakes and bearings all have to be 100% , because you're adding load to the fiver that it wasn't designed for .
How much does the boat and trailer weigh ?
Can you get the trailer to a set of scales , to get an accurate weight?
Will you be exceeding your trucks , GCWR ?