Have to agree on the possibility of vacuum being an issue. Your description of the spider web of vacuum lines into which the t-box is plugged is one clue. TH350's like their vacuum to be as clean and direct as possible. For that you might have to to the original source on the intake manifold and change the fitting for a multiport one that can feed the box plus your other suckers. The while you're there thing to do is check the rubber hoses that make the connections between the primary steel line and the vacuum port and module to make sure they aren't split or torn or rotten.
The other thing is that in the lockup 350 the lockup only occurs in third at a certain speed or rpm. The lower gears don't get that. I pulled an E/C version out of my Non G-Body--G-Body and plugged in a 700R4 so the lockup thing is a familiar issue for me.
You can actually visit the transmission and disconnect the electrical connection that is plugged into the driver's side of the box and the unit will continue to function quite happily. That actually might also be a test for you to try to see if the lockup is popping in and out during hill climbs. Auto-sticks will mostly always try to put themselves into a comfortable range or gear when dealing with hills so it downshifting under climb or if the up-hill speed slows down to a certain mph is to be expected.
Nick
The other thing is that in the lockup 350 the lockup only occurs in third at a certain speed or rpm. The lower gears don't get that. I pulled an E/C version out of my Non G-Body--G-Body and plugged in a 700R4 so the lockup thing is a familiar issue for me.
You can actually visit the transmission and disconnect the electrical connection that is plugged into the driver's side of the box and the unit will continue to function quite happily. That actually might also be a test for you to try to see if the lockup is popping in and out during hill climbs. Auto-sticks will mostly always try to put themselves into a comfortable range or gear when dealing with hills so it downshifting under climb or if the up-hill speed slows down to a certain mph is to be expected.
Nick