This thread prompted me to do a little research that I think might be of interest. My 1966 Lund C-14 (Not WC, which doesn't show up until the mid 80s) has a length of 13-8, 55" beam, weighs 160, and the plate shows a capacity of 960 and is rated at 25HP. In '69 Lund added 4.5" to the length, 5 pounds to the weight, and the capacity drops to 880 and the rated HP to 20. Throughout the next 14 years various minor tweaks are made, the capacity varies dropping as low as 780 and settling in at 807, and the rated HP at 17 by DOT standards or 20 by BIA. All with the same length, beam, depth and transom as the 69 model.
In '84 Lund replaces it with the WC-14 in both a 15" and 20" transom model. The comparable 15" transom model is 13-10 long, has a 63" beam (vs 55), is 3.5" deeper amidships and at the bow, has a capacity of 785 pounds, and is rated for 25HP.
So the same model number gets 4+ inches longer and loses about 20% of its capacity and HP rating. Then it's redesigned to get significantly wider and deeper, and ends up with essentially the same capacity as the older model and the HP rating of a model 20 years older and fairly significantly smaller. Similarly, the relatively unchanged over that period S-14 model is rated at 50HP and 1195 pounds in 1966, but by 1984 with the same dimensions it's rated at 35HP and 895 pounds.
Clearly the rating standards changed over this time. I'd be wary of assuming these older boats are comparable to newer models, and I'd even be wary of their rating plates. On the other hand, unlike Dr Johnny Fever, I don't think the Boat Cops are going to track you down for not abiding by current standards, so we do what we feel comfortable with. I just think this sort of information is interesting and useful in each person deciding what they're comfortable with.