My Transmission guy says there were many Improvements actually to the mid 70's Transmissions from the 60's. When I bought new torque convertors the Company I used actually updated the Torque Convertors for my 69' 340 using many new parts from the 90's. So it comes down to the quality of the Builder. I wouldn't pay more than $100 for the core though, since you will have to go thru it. I have seven or eight sitting here, but you are too far to ship economically.
