'73 marked the change in dash board design...

So no.. unless you have a '72 that is registered a '73, the early units won't work. You need '73 or later parts. Possibly well into the 240 range will work, but I'm not sure about that.

Being a '73, it's also a rather nasty job. Perhaps the worse thing you could want to do. I'd go for stop leak.. get the expensive stuff. Ground black pepper works well too. Dump a cup or two in there.

If you're in a warm climate, you can just bypass the heater core...

If it is the same as a 240, the following link will give you an idea as to what you're in for...

http://cleanflametrap.com/heater_core/

Good luck! I feel for you...

--
-Matt I ♥ my ♂