Actually, the 1970 Chevelle LS6 is considered the highest HP production car from that era. No Hemi was rated higher. And the LS6 is said to have been rated VERY conservatively for insurance purposes. Ive read tests where people have built an LS6 using all NOS parts, then tested it and found the HP numbers to be closer to the 650-700 range. Yes, an LS6 car was a brute.
Towards the end of the muscle cars they did, as I suggested above for the LS6. But actual ratings and/or measurements should be the same as then otherwise. Companies may rate them conservatively these days too, but they should be measuring the same way they did 30-40 years ago. There is no evidence that suggest an engine rated at 300hp these days is more powerful than one rated for 300hp back then. If anything, its more likely the other way around, and the older one was rated more conservatively. But again this depends on the actual situation, you cannot make a blanket statement like engines are 'more powerful' today then they were back then. (not that you said this flip, just sayin)
I would agree however that with new technology available, it is easier to squeeze more ponies out of each cubic inch. It wasn't too many decades ago 1Hp per cu.in. was considered the goal for performance. But now that's fairly easy to surpass. The difference is, the old cars tended to have huge engines that weren't tuned to their max, while today's engines tend to be smaller and tweaked out to their max. I prefer mixing the two.... new technology mated to lots of cubic inches...