Basically a 451 is a 440 in a slightly smaller package. As Mopar used such heavy parts, the first popular 451s, which were made with aftermarket lighter parts, revved quicker and felt stronger than the equivalent displacement mostly stock 440s.

One important thing to remember is cost. Way back when, people were just about giving 400 blocks away. I could have bought dozens of brand new 400 shortblocks in the crate for $400 or less, and used 400s were a $100 block. So when 451boy (AndyF) showed us how to use those cheap blocks, it didn't take long to catch on. And that happened just about the same time as the cheap parts started coming in from China. It was synchronicity. The only thing missing was cheap heads. That's when 440Source bit the financial bullet and developed the Stealth head.
They also talked about a block, but like so many other people have found out, blocks are much harder to cast than heads. So they wisely dropped the block idea, which probably saved the company.

R.