I can tell from your pictures that the block is a bit wider than a 351C. I'll go along with the block having a 351W deck height. That still explains the needs for offset pushrod cups, and the spacer for the 4 exhaust primaries.
If the engine was built circa 1997, and was equipped with A3 heads, it wasn't a current NASCAR engine, although the A3 heads & intake were most definitely "former" NASCAR parts.
$40K for the engine in 1997 is a wild number … not realistic. No wonder the seller couldn't provide a receipt.
The 3662 number stamped into the heads and block were most definitely the numbers of a well organized engine assembler. The question is who? If Roush won't respond there weren't many Pantera vendors that were suppliers for SVO parts. Two that I can think of, Hall Pantera and Pantera Performance Center of Colorado (Dennis Quella aka PPCC). Start with Tara at Hall Pantera. Dyno Don Nicholson was building engines back then too. Jim Kuntz might have been building street engines by then.
Keep an eye on the tappets … solid roller tappets are going to eventually fail; unless that engine has a custom made rev-kit. They normally fail within 10,000 miles or less. The solid tappet lash clearance allows the solid roller tappets to bounce (chatter or oscillate) within the clearance, and this beats the little needle bearings of the rollers to death.