Hay 47... I just finished installing a OD transmission in my 53. To recognise a 3 speed over drive transmission just look for the solenoid sticking out of the left side of the transmission (as viewed from the rear) and the govener sticking out of the right side near the rear of the trans.
You will have to go to an open drive shaft and rear end. I used a 55-1 rear end and housing with a mid 60's 3.90 pumpkin. This geves me a final OD drive of about 2.73 on the highway. Works great.
It was a lot of work but well worth the effort.
Tom

1953 Chevolet 5 window