HP - when was it used and why?

Car manufacturing operations were alot different 40 years ago than they are today. More manual labor, less automation and quality control. That's why today you can only get a select few trim combinations, tighly integrated with their corresponding powertrain packages. Back then if you knew the right guy in the sales dept. you could special order literally almost any package combination you wanted and have it custom built right on the assembly line. That led to alot of things like recordkeeping snafus, low-production runs and year crossover anomalies that makes genralizing about what came with what or what was available when more or less an art and a science.

There were alot of mostly followed package rules and a some pretty hard and fast rules. If the HP block is all you are talking about, one of the few things you can be just about absolutely sure of is an HP block left the factory with a 268 cam. Beyond that it's gonna be pretty variable along certain guidelines. There were even some things fleet managers could order from the factory like cop car builds most people never even heard about, including the dealers.