Mabel's line "Ben Franklin really WAS a woman" references an earlier episode in which Stan has the kids counterfeiting hundred dollar bills and criticizes them for making Franklin look like a woman.
First Episode of Gravity Falls that Soos Doesn't appear in however he is mentioned at one point.
A bust of Frederick the Great, Emperor of Prussia, is visible in the secret chamber, next to the book containing the truth about Quentin Trembley.
Stan's car model is "El Diablo" (the Devil), and a reference to the classical Cadillac Eldorado. The letters are visible when Stan is trying to get his car out of the mud.