For a 300 hp 289 in a '66 chassis the only thing I would consider a "need" would be a dual-circuit master cylinder, for the safety-aspect in case of failure of a line or wheel cylinder. The 3.03 3-speed transmission was the standard manual transmission behind many a FE and 351W/351C, the 8-inch rear is up to the task though you may want to add a LSD, like the Eaton-Detroit TrueTrac, nothing all that wrong with the drum brakes... many 4-wheel drummed 390's were sold.
You should consider the "Arning Drop" to improve your front suspension geometry, a quality "Export Brace" to stiffen the shock tower-to-cowl connection and, generally, replace any worn suspension components with quality (Moog) replacements. Other than that, whatever your heart (and wallet) desires.