According to my catalogs....single snorkel for 68 and dual for 69. Never seen an original 68 with dual snorkel air cleaner...
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YOU are correct. My bad. I recall a forum discussion on this and simply could not find it to reflect back on. However, I did finally find it: https://www.gtoforum.com/f178/68-400-ho-details-125401/
I had quickly referenced the Pontiac GTO Restoration Guide and read the text which did not clarify. Did not find a photo which I thought it had - but I just found it going through the book again. Single snorkel, chrome lid, painted 60% gloss.
The HO used the same air cleaner as any GTO, single snorkel with chrome lid. 1969 and up had the dual snorkel and Pinion head provided a very detailed description of the different types. The only fly in the ointment was the supposedly original 1968 GTO HO 400 written of in Hemmings which had a dual snorkel. It was never determined if the owner installed this himself along the way OR it may have been a very late 1968 build that got the 1969 dual snorkel air cleaner - 'cause Pontiac did things like this.
So check out the thread and this should clarify. Personally, dual snorkel would be on my engine if I were to run something other than an open element air cleaner which I prefer and run. Something about the visual balance of two snouts versus just the one.
A very interesting test was done by Engine Masters on air cleaner flow. One test included a dual snorkel air cleaner on a 750 HP engine. The dual snorkel showed a loss of 53 HP. When flipping the lid over, it lost only 11.9 HP. The best was an open velocity stack which increased HP by 5.7 HP and the fly in the ointment that came in as second best was a fitted salad bowl which increased HP by 3HP.
Keep in mind this 750 HP engine was no doubt a big breather and it spun some higher RPM's. I don't think you would see the same kind of losses on a lower RPM/HP type engine.