I do not think those will fit Mark, it will be close.
I bought some used heater boxes from a swap, cleaned them up, and put one on my 73to replace one that was rusty; found that the tube coming out of the box is of thicker diameter and does not mate up well to the muffler, it is darn close but not a good fit. - this box it seems came from a single port I tried it anyway.
Used the hard asbestos gasket to seal the tube from the box to the muffler as one normally would. In about 2 days the asbestos gasket broke into pieces from the vibration and poor fit. Replaced it with a metal mesh gasket, and that seems to be holding though there is a slight exhuast leak.
Is your car 74 stock engine? they may even have a diff muffler than mine...single exhaust?
Maybe it is my car....who knows