The folks I am buying the car from have owned it for 36 years, and they tell me they have never had it over 65 MPH, and rarely, if ever, have it on the freeway. They do not feel there has been any change over time as to the performance of the car. They were told about 10,000 miles ago that they had a couple cylinders test low, and they had the motor completely rebuilt at that time - and bored .030 over. Since then the carb was then rebuilt and rejetted and most recently a stiffer return spring intalled. All of these things are pointers, but according to them the performance has not changed before or after all of this. Then again, this is a retiring couple who have never had performance on their agenda for this car - it was a light duty occassional cruiser.
My initial gut says throttle hinderance of some sort, as I never felt like I got any RPM's, and the pedal was so stiff to depress - but I will check out all of your suggestions until I figure it out.