Did some digging today and found out some additional information. Talked to a gentleman in Illinois who was a close friend of Tom Hoover and worked in the racing industry during that time. Yes, there was such a thing as a D5 block for the D5 Heads. He had not seen a D5 block, but know they existed. He suggested talking to Ray Barton, he may know more about it.

Reached out to Ray and yes, they were a Pro Stock block for the experimental D5 head program and had less water jacket in the Intake pushrod area to allow for the relocated intake pushrod closer to the cylinder, thus the different casting number. He mentioned building a few blown drag boat engines using those blocks in the past and use to come across them from time to time. The block will accept regular Hemi heads without modifications, but was designed for D5 heads.

Now, just have to decide what makes sense for it...