I've been collecting Timelys since 1995, and I've seen a decent amount of Cap 78s in that time. I'd say it has moderate scarcity but not ultra tough. Here is the scope of Atlas books that were cover dated Sept 1954 (May 1954 real time). I'm sure there is a matching inside BC/outside BC on one of these books. Good thing these are all relatively cheap. The Torch 38 was out one month before Cap 78, so it could very well have a matching inside BC.
http://www.mikesamazingworld.com/mikes/features/newsstand.php?publisher=marvel&type=cover&month=9&year=1954&sort=alpha
At least you aren't dealing with a late 30s/early 1940 Johnson Smith & Co (Detroit, MI) back cover ad. Those ads have different dept. #'s so although the ad looks the same, the dept # could be different and you will have the wrong BC married.
Best to your hunt for the right BC.