Yes to the high probability of your t-mission being a TH350. As depicted in the picture above the "C" variant does come with the lockup provision. The plug for that socket has only two wires, and, as already noted, if it is disconnected or the harness is gone altogether, no biggie., the box will move from gear to gear without issues. I will leave it to a manual to explain exactly how the lock up works.
In the world of the first gen SBC's, starting around the middle 60's, the 350 was about as common and prevalent in GM's Chevrolet line of cars as pepperoni is on pizza. There is actually a whole subindustry attached to them in terms of parts and service and rebuild/performance products.
As for tracking the vacuum line, from the modulator valve it will proceed up to parallel the case and follow the shape of the case forward and upward to the seam between the t-case and the back of the engine block where the two bolt together. From there, depending on its source for vacuum, it can take a path to either side of the distributor and plug in to a vacuum port screwed into the intake manifold just ahead of the dizzy.
That port fitting has many shapes and sizes as it can possess as few as one port or as many as four or more, dependintg on many subsystems located in the vehicle make use of vacuum to operate.