I'd pass on the boat trailer conversion. Everyone needs to remember a axle rating doesn't mean it can carry that as much as cargo, the weight of trailer itself is included in that capacity, if the axle is rated at 3500 lbs and the trailer frame/flooring/fenders etc weigh 1000 lbs your load capacity is now down to 2500 lbs, most do not take this into account.
Always, always get more capactiy than you think you'll ever need. Most almost always wind up exceeding the trailers capacity.
Tandem axle is the only way to go for safety in my opinion, think of blowing a tire going fifty mile an hour with a cord and a half on the trailer, it'll whip your azz to death, could over turn the towing vehicle. On the farm we use many different trailers, gooseneck tandem duals, stock trailers, everything down to little 5' x 8' mesh floor trailer set up to pull our welding equipment. In the wood business, we use a 16' tandem trailer with 7500 lbs axles and 18" sides we can get a little over two cord on it safely with out any danger of wood falling off. We use an old 71 chevy half ton bed and frame converted to a trailer to pull wood out of areas that are too rough or confined to use longer trailers in, we can load a full cord on it, but it is overloaded, we've used it in this manner since 1978 and had no issues.
Around here you can buy new 16' tandem trailer's with 18" sides with two 3500 lbs axles for a 1000.00 bucks anyday of the weeks with ramps included.
Used anywhere from 450.00 to 800.00 bucks, shop around and get a good trailer, boat trailers are pretty dang light to start with.