I have a 69 Chevelle that origionaly had A/C, somebody removed the entire A/C and heater unit on both sides of the firewall. Before I installed my new bigblock (502) motor in the car I completely filled the hole in the firewall with a piece of welded in sheet metal. Now I have no opening. I want to put in a non A/C heater box on both sides. Does anybody have sizes or pictures of the opening? Do I need to get a heater box from a big block doner car or are the all the same?