I need help but am afraid I'll get shot

Sorry about this question but its another 833 decode.

I was at the autowrecker today and inquired about a 833 tranny, he said he has one but isn't sure if its a 4 spd or a 4sp OD tranny. He said it can from a 75 duster.

It has a cast iron case and the numbers on the ID pad are
1B235854
P833 3376

Some other numbers are

C-99126-3 on the Tailshaft
C-99386
C99129H can't remember where these last two numbers were located on the transmission!!

Could someone please help me decode this. If its worth buying I don't want to miss out.

Another question is how well the 4spd OD handles performance upgrades, my engine is 425-450 hp. Will the 4 spd OD live very long? and will it kill my performance? I have never driven a 4 spd OD so I don't know if there is too much gap between gears for decent performance?

Thanks

Brambles