What size tire came on the 1973 originally?

That is tough.  The original tires were bias-ply, and when radials came out, bias plys (and their unique sizing) went the way of the dinosaur.  There's not a direct swap, but I would suggest something like a 225-75R15 for starters.  Ask a local tire shop to be sure.

The 73 tire size is stated as GR70X15, 15", 70 Series. What I run on my 74 255/60/15 all around which is much wider then the original. As Littlehorn stated the 225/75R15 may be very close to the original dimensions.

The closes conversion is 225/70r15.  You can find more information here:  http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=45