I figure I'd chime in better late than never....
I finally went and pulled out some stuff to photograph, figured since I own a 68 390GT mustang and a 70 428CJ and have hoarded a pretty big pile of parts, maybe I could help.
Snorkles..
The 68 390 GT Mustang snorkles I have (one I bought NOS in 1979 with the ugly cougar piece I also photographed, one I bought later, but both are Identical, and match the beat to hell one that came with my car...) both sweep back like a 428CJ, but the snorkel is shorter overall. The flange on the bottom is the same size on both, but the 390 flange has 2 small circular notches cut in where the s tube seams line up.
The 390 GT snorkel I photographed has been repainted and restamped, but is definitely an original.
S Tubes.....
The 390 GT S tube "big end" is smaller, and fit's inside the flange instead of outside like the 428 setup. The seams are cut back so they don't hit the snorkel flange.
As for the other snorkel I photographed, it's sweeps forward and is marked C8WF-9A626-A and I assume it's the 390 2bbl part. Believe it or not, I bought that new from the dealer in 1979, when I opened the box, I found that stapled together mess, right from ford. When I went to return it, the Dealer gave me my money back and told me to keep it. It's been in my parts pile ever since. ( I still have the box.....) I figure I could fix it at some point if I ever needed it.
Hope this helps, John
EDIT...
I Just read through this whole post (after I posted....
), and realized there was some controversy over the forward vs rearward swept snorkel. I always assumed that the rearward one was correct, as the NOS one I bought was rearward swept, and when I bought my 68 390 car, it had what I'm sure was the original air cleaner with it that also had the rearward swept snorkel. The car was a really unmolested original 390 4 speed GT fastback, a really late (7/18/68) Metuchen built car that was involved in an accident in 1973 with 70K miles and hidden in a barn till I bought it. The only thing missing was the smog, The date on the air cleaner motor was consistant with the build date of the car (late may if I remember correctly). I still have the air cleaner and snorkel somewhere ( I never get rid of anything), I'll try and pull it out along with the NOS C8OF snorkel and take a few more pic's.( The original air cleaner was beat up by someone trying to open the hood after the accident with a prybar, They also broke the original distributor) Again I'm 99.9% certain that my car was built with a rearward swept snorkel as although the car was collision damaged it was pretty much unmolested, down to the original hoses and plug wires. The S tube I photographed was off my car ( It's been repainted by me...)
I wonder if anyone has one of the forward swept snorkles that still has it's ink stamped part number, I'm curious if it's the Cougar/2 bbl part or something else. I'm comfortable with using the swept back snorkel as it matches the one that I'm sure the car came with, and it matches the part that both the MPC and the assembly drawings call out.
Sorry to confuse things worse, John