Thanks for the replies. I must be missing something on the 4 bolt mains, but I only see the center 3 sets in the picture. My SCJ block had that same arrangement. Does a Boss block have a 4th set?
I will find out more about this block tomorrow, but based on the responses I think it is likely that it is just a standard block that has been fitted with screw-in core plugs. The guy who was building it also has a set of those Kaase Boss 429 heads, so probably it is dressed up to look like a Boss 429 block on the outside.