Classic/Antique Car Repair: poor performance, chevy truck, fuel filter

Question
i have a 1959 chevy truck with a 235 6, it has been a great truck, recently, it has no power under load, it will barely go 20 mph up hill, i checked the plugs and the spark at each plug, it idles perfectly? any help is appreciated

Answer
It could be any one of the following list of things:
Fuel filter
Bad Fuel pump
Plugged exhaust
Bad ignition coil
Low engine compression
Plugged air filter
I would start with a fuel filter and then is that does not do the trick test the other components.
Brad