Sir,
Not sure if they ever made a cast, stepped register conventional/straight four bolt main Chevy block? I would have to check most guys just use the splayed billet caps. I also would think that if you put straight caps on a block it would be prone to crack in the main web area in the cutout section. If you have a block laying around flip it over and you will see what I am talking about. I am out of town right now PM me your number and I will give you a call to talk it over.
Thanks,
Bob