I'd be curious to know what the plugs look like and/or what a monitor shows from the O2 sensors while this is happening. Also, I'm a little concerned about your comment about the car having a new distributor, since these cars don't have distributors.
Here you go. I can't remember, but I think some early 1996s had the 95 style ECU, and later ones had the 96-99 style ECU.https://www.2gnt.com/index.php?d=ECU_Pinout
How are you measuring AFR? Do you have a wideband O2 sensor? Narrowbands will oscillate like what you're describing. Otherwise, putting larger injectors in your car without a way to adjust the base fuel pressure can certainly cause you to run rich.
As far as I know, your two options are: 1) fabricate one from scratch, 2) port and polish a stock manifold. There used to be some Chinese-made sheet metal manifolds available. The quality was dubious, and I'm not sure they're still in production.
I agree that threads will strip before the torque affects the bearing. I actually can't find an exact value in the 420A overhaul manual, but the Neon FSM says the bolt should be torqued to 61 N-m (45 ft-lbs).
I've never owned an A/T 420A, but I'm fairly certain all transmission-related fuses are in the dedicated relay/fuse box in the engine bay. It shouldn't be too hard to track down the right one.
I would replace the whole thing. Welding it will take just as much work (removing the transmission, disassembly, etc.), and depending on who welds it, it will cost just as much, if not more.
You might have gotten this all straightened out by now, but here's what I see:FunctionInternal PinsConnector PinsIAC controlJS0-JS3IACxx12V for IAC controllerJS9 to S12CN/ABarometric pressure sensorJS5N/AGeneric/tach output (5V pull-up)JS11 to pad...