i think they are a tiny bit hevier but its not significant i don't think, and you should cheack for soft spots but they are neither in many older boats nor exclusivly in older boats, my old 1989 16 had a soft spot
The change date is the 1984 model year... class rules changed from 340 min to 320.
Older boats are more prone to delamination. They used to cut sheets from big blocks of clark foam. The foam was not consistant. Not sure the change date on that though. I had an '84 with a delam that I had to fix. Lasted for as long as we had contact with that boat.