Bill,
I guess I'm confused. Are you wanting to run both sets of belts at once? Or, just a lap belt at track events/autocrosses?
The location of the eyebolt on the '73 might not be in a good place for belt orientation or structural reasons.
There's a great write-up on the do's and don'ts of harness installation by the guys that sell Schroth harnesses here in the states. They might even have a belt set that would give you the benefits of a harness and the suitability for use in a car without a race seat and cage. I know they make a four-point harness that acts like a three-point belt in a wreck. It's worth looking at the site.
Go here:
http://www.hmsmotorsport.com/
JR