depends how much you wanna spend! a simple upgrade form a 2 barrell to a 4 is nice. then you could do full exhaust with headers, then gears (might be outta order but you get the point).
i personally have made many threads on this subject so have a gander in the search area.
Upgrade the gears, headers, 4bbl and performance intake, good heads, a performance cam, and a nice exhaust with a H or X pipe should make it go.
HTH
--Ky;e
Ditto to 5.0ina66. This willgive you a good start. If your serious about power Spend money on good heads, research the diffrence between dual/single plane intake, calculate hwy rpms for diffrent rearend gear ratios and know what you'd be comfortable with, research camshaft geometry (can 'o' worms). Find the diffrence between vaccum/mechanical secondaries on a 4bbl carb (600 CFM will be plenty of carb for a 289). One of the best bang for your buck mods to do for your engine would be to get rid of that crappy piont and condenser ignition and install a petronics module, you will be happy you did.
Go with vacuum secondaries for a street car. Mechanicals are normally used for race applications, though it doesnt matter other than mechanical will burn more gas.