However does this negate the issue of small block-non GT cars (302 4bbl) cars getting the single type exhaust?
It doesn't, never said small blocks did not get single exhaust.
As noted stock was single, even on the J codes, but I believe dual was an option (as if you could get it painted any Ford color you wanted I would be willing to bet at least a few J code buyers opted for dual exhaust.... but could be wrong)
My understanding of the BOSE is a "line filler" slotted in at the plant. I have heard a few possibilities why, but similar reasons:
1. Ford was running a series of cars, I have heard they tried to batch similar cars to ease differences car-to-car on the line, and needed to keep the line going so had someone tick some boxes to make an order to fill some blanks on the line to meet quotas for the day.
2. Similar to above, but just cars to fill a slot in production, dealer/stock orders did not keep up with the line quantity, so someone had to make up options.
3. Again similar, but Ford was looking at production runs of parts they needed to use up and filled the line accordingly.
I don't know which is the reality, or if all have grains of truth. I'd lean towards #2 as I am not sure cars were batched by body style/engine, and #3 seems odd for a Dec '67 car as one would think later in the year finishing off on-hand stock would have been more likely... if it even happened.
As for how many ask Kevin Marti for a statistical report, not sure if he'd do "non GT S codes and build date by plant" but you could ask... if you really want to know. They are more common that folks think, I've seen quite a few over the years. I'd guess 1/4 to 1/3 of all S codes were non GT, but that is a pretty wild guess...
One odd factoid I have come across (as S codes are dear to me) is no 68 S code convertibles got a bench seat.