Headers and exhaust will certainly help, but probably not going to get you where you want. Depending on the article you read, Vortec heads will add 25-50HP over the 70's smog era heads. The Edlebrock intake will tack on a few HP and torque numbers, but its not huge.
The number 1 factor in engine performance will be the cam shaft. Cams are somewhat flexible in what springs they are compatible with and mild to moderate cams will use springs in the same ballpark. I would assume the springs on your motor are pretty high mileage and borderline worn out. Too light of a spring and the valves float and you loose power. Too stiff of a spring and you flatten your lifters and cam.
Replacing the cam, properly, is quite a task. You will need, cam, lifters (you can never ever ever ever re-use flat tappet lifters, even if the engine has 1 mile on it), distributor gear, new timing gear set, you might need different length pushrods (if the base circle of the cam is a different diameter, or lifters are a different height). Cam shaft manufacturers not only recommend the proper spring to be used with the cam, but also the installed height of the spring. More than likely you have valve spring seat shims installed now to bring them to the proper installed height. On a high mileage engine these wear (on the heads I tore apart the seat was almost completely destroyed) and that changes the installed height. Then you have all the misc gaskets and labor to replace the cam itself. Fortunately you do not have to replace the cam bearings unless they are worn, do you know the history of the engine? Your oil pressure would be an indication of bearing wear.