Ancient History: when Ford walked away from the Pantera project 'sometime' in 1974 (date varies with the source), DeTomaso began doing what he'd always done with non-U.S Panteras sold direct since '71: they were offered from Modena with all sorts of upgrades, options and enhancements never seen in Ford-marketed cars. One was higher-power versions of whatever engine the factory was offering at the time. This included Weber intakes & carb sets, Holley carbs, different cams & higher compression. In the '80s, regeared ZFs were popular. Some of the enhancements offered appear to be Speed-shop mods sourced from California and installed by the factory.
So I would NOT be surprised to find a factory-paperwork-verified Pantera (351-C or 351-W) with anything up to (but maybe not including) a stroker crank- which was uncommon in street engines in the early days. Also keep in mind that some owners used the factory as a Service Center for their cars during the '70s-'80s and had routine or even major work done there. So I could believe an overbored block, Boss heads, forged pistons, a reground crank or regeared ZF etc from DeTomaso as a freshening or repair.
Note also that a few Euro-owners of such 'enhanced' powertrains bought new- sourced through concessionaires for each country- have been rumored to occasionally discover a stone-stock motor inside, even though their original bill of sale lists a '350 bhp (or more) engine'. What happened? Don't know; nearly all the participants have passed on and 30-40 years ago is a LONG time ago to try to trace out such details on a limited production, essentially hand- built sports GT. I realize this doesn't clarify things much for you- sorry.