Most full size cars of that era used P225/75R15-size tires
Please note: The tires on my 1969 Buick Electra 225 are 205/70R15 and they are on the factory rims that came with the car.
Underneath the car behind the rear axle
According to Haynes: Warning: Engine must be completely cool prior to doing this! 1. Disconnect negative battery cable. 2. Drain the cooling system. 3. Disconnect the heater hoses from the heater core tubes @ firewall. 4. Remove heater case nuts from engine side of firewall. 5. Remove dashboard trim panels and the instrument panel pad. 6. Detatch lower air duct & defroster duct from heater unit. 7. Disconnect & label everything from heater unit. (label so you know where to put them back to) 8.remove the heater case-to-dash nuts/bolts & pull case back until studs clear the firewall, remove entire unit from vehicle. 9. Seperate heater case, remove core retaining screws & remove core from case. 10. Installation is reverse of removal. Make sure any damaged seals are replaced & seal the case with caulking. Good Luck!