It may help to treat this matter as two issues.

1) Engine performance.
The first thing to check is ignition timing. Do you have access to a timing light, and if so are you familiar with the use of one?
Second, in addition to the tank screen, there should be an inline fuel filter between the pump and the carburetor. If there was rust in the tank screen (common), the inline filter may be clogged.

2) Vacuum
An engine that is operating with retarded ignition timing will have very poor manifold vacuum, and hence the supply to the chassis will also be poor.
The chassis vacuum system, being composed of a number of plastic pipes connected by short sections of rubber tubing, offers many points for leaks. All of those pieces of forty year old rubber tubing are prime suspects for leaks.