I did read long ago harmonics at certain rpm ranges were a pretty big issue in long-running engines (think Indy 500, F-1, NASCAR).
If any Ford designed or Ford paid-for engines could have used a flat plane crank, it was a.) the 255 CID 4-cam Indy car engine (in many later permutations, up to 900 hp in turbo form, b.) the all-conquering 3.0 liter Cosworth Ford 4-cam F-1 (and FIA distance race) and much later c.) various turbo in-line 4 cylinder, Cosworth-headed FIA sports car and late Trans-Am Mustangs (Tom Gloy's cars).
Yet, IIRC none not only never had a flat crank but none were even talked about. Maybe Ford's engineers passed for a variety of reasons.