Put a floor jack under the center of the axle. Block the front tires. Jack it up. With the emergency brake off and the car in park and both tires in the air, try to spin one of the tires by hand.
If it doesn't spin freely, you have a posi.
Turbo 400's don't use the same kickdown process as the factory trans (200-4R, TH350's, 700R4's use a TV cable). TH400's use a switch. You need a TH400 kickdown kit. Check summitracing.
take a look at the pan on the transmission
Turbos and nitrous. Preferably at the same time.
Repeatability may be a slight issue.
Th200c (3 speed) pan is shaped like the TH350. It's a TH200r4.Isn't that a 200c? Not the 200r4
The metric on the pan gives it away
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