If the availability of baseball cards in Canada in the late 70's is any indication, even "national" distribution would have been very spotty at best.
On our cross country trip with a couple detours into Canada I bought 78OPC in the eastern part, near Niagara falls somewhere. Our route was Niagara up over the lakes to Sault St Marie. There was barely anything along that route at all. Many towns were double digit population. In the Canadian side of Glacier park they had cards in the gift shop
1977 OPC! When I asked if they had any 78's they told me they'd just gotten the cards a couple days before and were a bit surprise they'd gotten year old cards.
I'd have to think that Goudey was marketed nationwide, any candy wholesaler who ordered from them would have gotten a letter or ad of some sort. But it's entirely possible that individual wholesalers far enough away from a major league city didn't bother ordering any. The same would probably be true for any stores that were buying directly.
I wonder if any of Goudeys internal paperwork survived? Customer lists mailing lists that sort of stuff.
Steve B