I don't think your TH400 cross member is factory installed on a original 6 cyl car.

The original should be a stamped steel. Like this?

I know my 6 cyl 69 Camaro cross member is just like the ebay one.

I dont think you could get a TH400 w/a 6 cyl engine, but anything is possible I suppose.
Or Pontiac had them outfitted differently?

That trans should really wake up that 6. Great choice!

12 port SDS EFI