1964 Pontiac grand prix

Can the previous owner not answer these questions? A typical disc brake conversion kit usually means to a GM 11 inch rotor, OR the more performance oriented 13 inch slotted rotor. If you have 15 inch wheels then you have an 11 inch rotor. As for the steering box, GM used the same across Buick, Olds, Pontiac, the pitman arm nut should be 7/8" x 14, with a socket size of 1 5/16" No idea if yours has something aftermarket on it or not. For tranny, look for stamped codes or metal tags on housing. Automatics on top of housing, manuals on case or servo cover. Same thing with engine, look for casting numbers to get an idea what year it was made. You really should get in touch with whoever built the car to determine the parts used, hopefully previous owner can help you out there.

Thank you for the information. I can't ask the previous owner. We bought it from a dealer and they didn't have any clue about anything either. I guess that they can't release any information about previous owners to us. We have 16" wheels and I haven't needed to or had time to measure the rotors yet. I'm under the car now. The 2 stamps I see on the transmission are: MD8 and 8642547-4. We have an 1" play in the coupler of the steering gear box which relates to 5" in the steering wheel. Could it be just the coupler or both the coupler and gear box? Thanks again for your time and help.

I think you were right to suspect a non original transmission, as that appears to be a 700R4 probably from the 1980s. It is a really common swap hot rodders make to older cars, at least when you can't find a period correct original for a 60s or 70s car. There are tons of these transmissions in the bone yards. If it was properly built it will make a good upgrade, on the other hand, if it was a boneyard junker...