it could also be that the valve springs are either not right for the cam you have or are just flat worn out. i've seen a valve spring change add more than 50hp before, of course that was at higher RPM but still. i still have a feeling that the cam is either stock or too mild for what you're wanting.
at some point i'd recommend pulling the cam and identifying it, check the valve springs out and possibly just change them, especially if you find the cam is stock and decide to replace it.
here is what i would do for a start at least:
carb work, re-jet and possibly change squirters, power valves, etc. to optimize the carb for your app.
either re-curve distributor and add some kind of ignition box, MSD, Mallory, etc. or just ditch the dizzy and replace with an MSD unit and a 6a box.
change the oil to a low weight synthetic, like Mobil 1, Royal Purple, Valvoline, etc. 10w-30, same for trans and rear diff.
still don't know if you are running headers or not. if not i strongly suggest adding a set. either long tubes or a mid length shorty. i have hedman mid length shorties and really like them.
i still think your cam is going to be a limiting factor. it doesn't sound as though it has anything more than a stock cam or possibly a very mild RV style cam. consider changing the cam for something more along the lines of an X-303 cam or similar.