I 100% whole heartedly disagree with you. Back then, year is definitely important to a car. There were so many changes going on that there is a ton of year only stuff that means more performance one year than another. Not only that, but visual changes and all sorts of crap like that. So much was being changed even down to the day and month that it's important, and that comes in when you try to put a value on a care, and a rare one especially. How do you think they find out what a car is worth when someone "restores" one? An appraiser comes in and you always hear "period correct" or "that's not right for that year" even down to what freaking decals/stickers the damn car had on the hood or rear fender! Back then it was a sight to be seen when a car looked the same longer than 3 years.
Not to mention how many different engines were put into muscle cars, sometimes 10 or more were offered just in one vehicle, and that's not including various differences between ones offered.
For example, I'll use Ford here. A 1969 351W head some of the best factory heads you could get, among a ton of changes like firing order, cam specs, timing advance, compression ratio changes internals, etc. 2-3 years later that motor was a freaking pig that made no power in comparison. Year is DEFINITELY important with muscle cars, especially if you are into building with stock parts. You can make a ton of power with '90s GT40P heads but you should know there's a new spark plug angle that will make finding headers that fit tough for it. These heads are some of the closest you'll find to those good ones from the '60s (stock), anything else you need to stay away from or you'll choke your engine to death.
Even with AMC, the 390 had an uber high compression ratio of 12.3:1 or higher in some years and other years it was down to 8.5:1 or lower, but used the same engine block. Year is important.
Though that car was based on '70 AMC Javelin AMX.
