I was only able to get a 4" speaker in my door panel because of a trim strip about 4" from the bottom of the door panel. There is a place in the middle of the top of the dashboard where you might be able to install them - but I don't think it is well suited to larger speakers (I have not tried but I've been in the dash a lot recently and there is not a lot of space up there). There was an ad somewhere for kickpanels with speaker mounts for Mustangs, but I do not know if that applies to 71 to 73s. I'd say the best spot to try (easiest anyway) is the front lower corners of the doors. But even so - there ain't much room in there.
If you come up with something better - please post!! My system is good, but not great because of the speaker locations (it is hard to push sound with the top down and 2 1/2" exhaust with headers...)