Swapping out manual to C4 auto in 1967 mustang?
First of all... I gotta correct you on something.
the torque converter does NOT bolt up to the flex plate. It bolts up to the "flywheel" --- that bolts up to the crankshaft at the back of the engine, nearer to it's center. The flywheel is the big round part with the teeth (or gears) that go all around the outside. Your starter kicks out a drive gear that meshes with the flywheel's teeth to turn the engine over.
The flex plate is a steel plate that's in between the engine block and the transmission bell-housing.
You are correct, the inspection plate is a cover that bolts at the bottom of the bell-housing. AND that's it. It has a couple of bolts that hold it into place. It doesn't go in between ANYTHING.
You put the inspection cover on - AFTER - you bolt up (and tighten) the torque converters nuts to the flywheel, on the studs that are on the torque converter. And also (of course) the bolts that connect the engine to the tranny bell-housing.
When you drop the engine in, you put the flex plate on either; the tranny bell-housing OR the back of the engine block, depending on which one has the guide pins for the flex plate (I think it's the engine block). You spin the torque converter around to line up the studs on it with the holes in the flywheel so they match up.
This whole process is MUCH easier if you pull the tranny with the engine, then assemble them together... Then drop in the engine/transmission in as an entire unit. It requires a little more of this and that BUT it beats trying to line everything up, shaking the engine around and taking a chance of bending your flex plate, cracking the tranny bell-houising or damaging the torque converter/flywheel in the process........ Especially IF you're gonna pull the tranny anyway.