Sounds like you're happy with the way it runs after the dyno tune but not happy with the numbers. If the number was 325 horses would you still want more performance?
I took my engine to Mid-Island machine shop to dyno tune after I built it. The guys there told me they don't sell horsepower, when I asked them what they meant by that, they said they calibrate their dyno to give as close to real numbers as possible. One of the flat track racers from Victoria took his engine to a shop with the exact same dyno, after running there he took the engine to Mid-Island, the output was 15% lower at Mid -island. Same engine, same dyno, different calibration. They could have calibrated theirs to get me a higher number. I wasn't there to prove I could build and get great numbers, just to break in and tune. A dyno sheet with high HP numbers may be handy when it comes time to sell, but it doesn't really mean anything, it's how the car performs that counts.
One cannot plan or start modifications for more power until you know what you have now. That engine is 47 years old, unless you've owned it since new you don't know what's inside. If you're determined to overhaul or rebuild yours to get more power, you'll have to pull it out of the car, dismantle, measure every component and all clearances. Then you know what you have and what you will need to get the power output you are looking for. A few manufacturers boast [fill in the blank] more horsepower right out of the box, for their heads, cams, carbs etc.. But a high flowing head won't do you much good if you have deep dish pistons with a huge deck height and a smog cam. But if you're happy with the way it runs now, save the money for gas and drive it.
