Should I Replace AC Condenser?

I have run the original condenser on all my cars I restored. The only one that gave/gives me fits is a '73 Dart system. It has crazy high head pressure once the temps get above 90F and the low side is _always_ below 15PSI no matter what you do to the system. It is so bad the clutch can't engage if it cycles and will burn up the clutch. I have tried everything including putting in a large 134 condenser. Did nothing. So my experience is maybe putting the R134 condenser into a system that works reasonably well will make it work better but if the system is working like crap it won't magically fix it. I have recently found that there was a production change mid way through '73 (this was the first year for and updated system from the '70-'72 systems) and the evaporators are radically different. You can't even get a new evaporator for the early '73s. I have a '74 box that I plan on grafting in temporarily one of these years to see it that gets the pressures down to a more correct place. My hope is they realized they had a bad design and fixed in with the update and I can put the '74 box in for the '73.