I did the same thing on finding seat placement on a hull but in water. I had the hull glassed on both sides and went to a pond and had a few people watch from the side as I slid forward and back to get the most neutral position. Then just marked it with tape.

You right. - all you have to do is go on line and look at 14' boats to see the weight carrying capacity and check the width of the hulls. Granted, hull shape matters but at least you won't be way too narrow for the load.