you want to connect the vacuum advance on the 71-73 mustangs to the ported vacuum port.
it had nothing to do with the compression of the motor it has to do with the configuration of the mechanical advance in the stock distributor and the emissions laws at the time.
at Idle rpms the carburetor is configured so that vacuum is 0"HG through the ported port. this turns off the vacuum advance at idle. this means at idle only the mechanical advance is working which at idle rpms should be adding 0 degrees of advance so the motor is only using the set initial advance which stock is between 6-8 degrees. this lower timing forced the exhaust temperature to be hotter, thus at Idle the emissions from the car was reduced due to higher temperature burn off of the car exhaust.
you can run the car off full manifold vacuum if you want want, what will happen is at idle the vacuum advance will now be turned on since the vacuum at idle can be as high as 25"HG the advance might be fully on, this will significantly raise the idle RPMs and change the tune of the car, you may have too much timing for the range of throttle 0-30mph.
so you would have to retune the car to use full manifold vacuum.
At idle your exhaust temperature will be low you will also have more timing this will make the car more punchy and have more power off the line, because of all the adding motor timing advance. you will lose some MPG and when the car runs hot it will get very bitchy and be a pain in the butt when sitting in bumper to bumper traffic.
there is another thing ported verse full manifold vacuum was used for.
If your car had air conditioning Full manifold vacuum to the vacuum advance was used as a engine over heat fail-safe.
i wrote an article about the Distributor Vacuum control valve.
http://www.7173mustangs.com/thread-under-the-hood-mysteries-distributor-vaccum-control-valve
basically this was a heat sensor switch that changed the vacuum going to the vacuum advance in the case of engine overheating at idle.
when a A/C car would overheat sitting in traffic the DVCV would turn on switching ported vacuum to full manifold vacuum at idle.
this would raise Engine RPMS and increase the cooling Fan speed of the motor, this would cool down the radiator faster thus lowering engine temperature which would cool the motor and turn off the DVCV returning the car to ported vaccum at idle. it was sort of like an emergency overheat fail safe to protect the motor.
many people ripped the DVCV off the cars thinking it was part of the evil emissions system. It sort of was but it was a good thing.