Holley secondaries won't open

A 4 barrel w/intake, an mild cam and dual exhaust will make a noticeable difference on a 318. Something like a Comp XE262. Has the engine ever been apart? If not, you want to replace the timing chain and cam sprocket ASAP.

A 600 CFM carb is probably enough for any mild 318. Sure the 750 will work, but it won't be any faster.