Classic/Antique Car Repair: 63 plymouth, vacuum gauge, windshield wipers

Question
i have a 63 valiant with a slant 6 (170 i think) and it suddenly had problems accelerating and increased gas consumption. oil was also dripping out of the oil fill cap (which is ventilated). i was using the car to drive to work (60 miles) each way 4 days a week because it ran so well and got great gas mileage. during the same time as the other problems, the muffler either imploded or exploded and developed a large hole. i replaced the muffler with a free flowing cherry-bomb type assuming the problem would be fixed but it didn't do much. i brought the car to a local mechanic and he suggested that a valve job was needed. i believed him until i ran into a retired mechanic who insists the problem is a collapsed exhaust pipe. he never saw my car but has worked on many in the past and he says it's a common problem. what do you think?

Answer
Sounds like valves to me. Put a vacuum gauge on your engine and see if it holds steady or swings like windshield wipers. The latter is a dead give away of bad valves.

Exhaust blockage is very detectable. Once the exhaust system is pressured up in front of the blockage, you will not be able to increase speed one mph, not just 'have problems accelerating'.