Here's an example of a 302 from the AFR website that is rather mild (except for the Victor Jr single plane intake maybe):
302ci, 165cc AFR heads; 10:1 CR, 650 cfm Speed Demon, Victor Jr. intake; MSD Digital 7, 36° Timing, Comp Cams 266, .544"/.555" lift 216/224 @.050, 112 Lobe Sep.; 1 5/8" headers; 92 Octane;
402 hp at 5700 rpm
373 lbs ft at 5000 rpm
300+ lbs ft from 2000-6000 rpm
Assuming that AFR is not exaggerating too much, I guess that would qualify as a 400 hp engine with at least decent street manners. Of course, it's a 302 rather than a 289, but an equaly equiped 289 would not be far behind (5% if it makes the same amount of hp/CID)
So, make that 302 a 289 (-5%), put it on a chassis dyno (-15%) and you end up with 324 hp. Considering the test is an advertisement and that the engine lacks some accessories and a full exhaust as well, the output once in a car may be around 300 hp at the wheels. I can assure you it will be a blast to drive.