Probably the modulator valve line. There may be a canister of sorts on the side of the trans with a line attached to it, if so, hook the other end to ported vacuum.
X a bunch but not ported because they run off full manifold vacuum, and if it smokes like a pig and won't shift the modulator is probably why.
When the diaphragm in it goes bad it lets trans fluid get sucked up into the intake.
67 is probably a 2 speed powerglide trans though unless it's been changed.