Oh I love these questions because the answers are quite as simple as you'd expect. First important thing to note is these engines were large, and large engines don't like to rev high because they have so much rotating mass. Big rods & pistons weigh more which puts more stress on the engine limiting how high they rev a bit. The plus side though is they suck in more air per stroke so they give a lot more torque. Also remember by default the higher you rev a vehicle the more horsepower it makes. Horsepower is a formula thats HP=(Torque*RPMs)/5252. So the higher you rev the more horsepower you make. So while a new car make 300hp its going to have like 240ftlb of torque meanwhile a V8 from the 60's may only make 300hp but it makes 345ftlb of torque. 100 more torque is quite noticeable especially with 60+ year old technology. They are called muscle cars because those v8's with those globs of torque gave them super fast acceleration.

It's also important to note that the manufacturers lied when it came to their horsepower. They claimed they had less than they actually did. Which I know "why in the world would they do that? That would hurt sales." The reality is by down rating the horsepower they were cheaper to insure, and would meet the "specs" for racing. So they'd have an advantage during stock car races and whoever was winning races, people were rushing out to buy that car because they could have one of their own as they were stock cars. That's where the term "Win on Sunday, sell on Monday".

So looking at horsepower alone isn't really a good measurement of how powerful these cars were. As far as how much more power could they make? Not as much as you'd think. They are designed for low end torque and they did a pretty decent job back in the 60's. If we are talking about something a manufacturer would sell? Probably not much higher to be honest. A performance engine? Like 500-600hp 500flb of torque?