[unruhjonny]

If the stampings look legit, I’d really be curious to hear the thoughts from someone authoritative on this.

An XH block used in either the 1968 or 1969 model year should be a 9790071 block.

If I have this straight, the 1967-1968 RAI block used a 2-bolt engine with cast slugs just like everything else made before the introduction of the ‘2506 block with the RAII;
That is to say that all 1968, and 1969 D-port 400’s would have used the 9790071 short block assembly.

Only the 1968.5 RAII & 1969 RAIV used the 9792506 casting, which was finished with four bolt mains, and assembled with forged (TRW) slugs.

With a change in how PMD was dealing with substitution or SR short blocks starting around 1970, it would not seem to be a stretch to think that a ‘2506 casting could have been put in service as a ‘0071 block…

The difference between 1969 & 1970 was that for 1969 only ‘2506 blocks were drilled and tapped for four bolt mains - while for 1970, the few 400’s NOT drilled and tapped are the oddities.

Could PMD have started substituting blocks assemblies before 1970?

I think it’s quite feasible.

If PMD did substitute, would the casting number have been wiped?

I think that everything I have learned of PMD’s practices would suggest that prior to 1970 the number of the block was of upmost importance..