Unfortunately, there's only one way to do it...
There is no "one size fits all Legend 35s" (or any other boat) ...because Hunter--all boat builders--restyle boats and redesign spaces with each new model year...sometimes in mid-model year. So a tank that fits another model year Legend 35s won't necessarily fit your '87 Legend...nor is the tank on the current Legend 35 (if there is one) likely to fit either. The only thing you can do is measure the dimensions of the space yourself, and go through the catalog to find the closest match, making sure you leave enough room for the hoses and fittings. Then you supply a copy of the catalog drawing, with the fitting locations and sizes indicated on it, to the mfr.