Ken, good question. I recall reading once that Ferrari got around the rules by claiming the GTO was simply a variant of the 250GT, a model that apparently was already homologated for GT class racing. However, I also found this:
http://en.wikipedia.org/wiki/Ferrari_250_GTO
that claims 100 examples were required for GT class racing and Ferrari skirted the rules by claiming production of GTOs that didn't actually exist and by numbering the chassis' out of sequence so as to appear to indicate more built than actually were. Only 39 GTOs were actually produced in total between '62 & '64.
I have also recognized (in this thread I think) that one would have had to have been brain dead not to realize a 389 would readily adapt to a chassis that accommodated the 326.
But IIRC, Pontiac did not install their own V8 into the Tempest chassis until '63 (and at that, it was the 337 CID configuration, only called a 326 to pass Corp. muster).
I'm certain PMD Engineering would have been developing the Tempest chassis during the '62 Model Year for the planned '63 small displacement Pontiac V8 offering.
It just seems they would have been getting ahead of themselves to pursue FIA homologation for the Tempest with 389 in '62.
Of course, we know they actually built a small handful of '63 Tempests with the 421SD engines. So contemplating a '62 389 Tempest would not have been out of the question.
I've just never come across that story before.
I hope somebody can dig up more of the story. Perhaps the topic needs to be pursued in the '61-'63 Tempest Forum?
As far as what guys on the street knew, one can easily suppose that if the GTO option had not been offered, quite a few '64 Lemans' would have gotten a transplanted 389 or 421 anyway. No question, lots of folks inside and outside PMD would have understood what an easy swap this would be. One thing that might have thwarted them a bit, had the factory not planned for it, in '64 the big car was still using the bellhousing mounted starter and Borg Warner 4 speed. The A body used the block mounted starter and the Muncie 4 speed. Since the A body was planned to receive the 389, the '64 389 blocks were designed to accept the block mounted starter even though the vast majority went into big Pontiacs. Had they not had that accommodation, the swap might have been a little more complicated I suppose.