Small Block & Big Block Chevy Specific - high perf blocks?

Did they stamp High Perf blocks in the 70s? And whats different about a high perf block?

They never stamped them. Big Blocks had some sort of HI PERF PASS cast into them but it really means nothing on the block. Almost every big block I've seen had HI PERF PASS cast into it even if it originally came in a motorhome or truck.
It doesn't matter if it was a two or four bolt or anything. There were also MAR, MARINE castings, and TRK and TRUCK castings. Usually the truck castings were tall decks, I don't know about the marine. I even pulled two Mercury Marine engines out of a boat that had the merc marine tag riveted to them but they said HI PERF PASS on the blocks.

Now the heads are a different story. Most heads that say HI PERF anything on them are rectangular port.

Nope, nothing special about a SBC or BBC asides from casting numbers for a numbers matching restoration.