'71 Chevy C/20 bogs and falls on it's face under a load. Idles fine. HELP!!!!?

sound,s like a vacuum leak or bad exhaust gas recirculation valve [ E.G.R.] ...remove the valve & check for carbon deposit,s as well as diaphragm integrity. check all vacuum line,s,hose,s including brake booster hose & valve. a vacuum gage will help. check timing as well as mechanical fly weight timing advance in your distributor. check all gasket,s that seal off vacuum..especially the intake manifold gasket..squirt some oil around the mating surface while running..if it suck,s it in you have a faulty gasket....also double check valve rocker adjustment,s...if your running stock ignition...replace cap,rotor , point,s,condenser...this will give you Access to the fly weight,s for the mechanical advance so you can be sure their free & there are no loose spring,s...best of luck!..edit..to all that answered a possible cam shaft because there was a problem with soft metal shaft,s...that was back a long time ago & that would not cause a backfire...the lobe,s would wear down preventing the cylinder from recieving a proper air/fuel charge...then close the valve early...this would account for lack of power but not a backfire situation.