When talking 4 stroke, it's about weight. and the size of your pontoons. When talking an older boat, normally you're talking smaller pontoons that can be overwhelmed by extra weight pretty easily. Point being, even if the boat is rated for 75hp, you may want to consider what a 75hp 4 stroke weighs.
The other thing is the engines may have several hp ratings for a given size engine. I know the Honda 35, 40, 45, 50, and the new 60 for instance, all use the same block and weigh within just a few pounds of each other from top to bottom. Other engine lines likely have something similar going on, with multiple hp ratings that all weigh the same. Point here is, you want to find where those big jumps in weight occur in the line of engines you're looking at.
With Honda for instance, the 35-60hp range weighs a little over 200lbs(220 or so). When you move up to 75hp, that engine will weigh closer to 400lbs. That much difference, at the very rear of the boat, can make quite a bit of difference in how the boat sits in the water - especially the boats with smaller tubes.
There's a reason the 40 and 50 hp 4 strokes are so popular on older pontoons......