Increasing the airflow through the engine will help, but it comes at a cost, you will lose low end torque. Splitting the exhaust manifold is about the cheapest way to go, not sure if you can keep the heat riser operative, though.
More carburation will help, too. You have some options. Simplest is to go to a two barrel, two stage carb, the Holley-Weber. Will still keep the torque up, and will help open up the top end. With different manifolds you can go with dual Rochester ones, dual Holley-Weber twos, or a single four.
Another area to improve is the ignition. If your distributor isn't worn out, you can add in a Pertronix electronic ignition for about $85. A GM HEI from a newer six can be adapted, but they are getting hard to find. Mallory Unilite is good, but expensive. Another option that can be used with or without HEI is an MSD-5 or -6 box, which will give multiple sparks every time a cylinder fires.
Now, if you have any money left
we can discuss cams.
