I'll assume the engine has been recently rebuilt to stock, or has fairly low miles. Pumpiing up a tired engine is kind of like kissing your sister (you not me, I might like it
If you only have money for one thing, I would suggest a set of headers. I always work backwards for performance bolt-ons ~ Headers - Intake - Carb. A Big Carb, and a crappy intake is a waste. Big Carb, good intake, stock exhaust, is also a waste. The faster you get rid of spent fuel, the quicker you can fill it up again. Figure 10-15% per item. Good Luck.