You say it runs just fine , If thats the case a rebuild will do little . You cannot get huge power from the 235 but it easy to get more than it came from the factory with . The easy way is to put more carburation on it , I went to a dual single barrel carb setup and had a very noticeable improvement , There are also manifolds so you can install a single 2 or 4 barrel carb and I think that might be a little easyer one to tune but the dual singles werent that bad .A set of headers will also help
When I reuilt my 235 I did alot to add power , dual carbs ,performance cam ,milled head , headers just to top the list . I did see a power improvement but it will never be a screamer . Each horsepower gets more $$
For real power and to keep the 6 cylinder look just go straight to a 292 but it still has its limitations and will not be cheap to get horsepower from . For many years the cheapest way to some horserpower has been in the small block V8. I just dont think I need a v-8 in my ol truck , I do run a 425Hp motor in 63 nova though
