The later blocks have lower nickle content as well.
If I were going to build a 600 HP motor with 8-10k invested in rods and pistons, I'd opt for an early standard bore 400 of any code, including a Catalina or Lemons. 67-69 GTO or Firebird block and crank $800. Same year Cat, Lemans or GP? $300-400. Same block, just not a desirable correct "code".
It's piece of mind, insurance and assurance of my investment of machining and parts.
