How much horsepower should I expect?

IMHO, even with the bigger/racier carb and headers it's probably a 425-450 HP motor.

BTW that's a lot! It will fry those 215/70/15 tires at will with 3.55 and low gearset in the 518 trans. You're just loosing a little HP with carb and exhaust.

The Comp 268 roller: 268/274, 230/236 @.050, .552/.564", and 110°

Should be nice and street friendly with a 4" stroke and 230/236 @.050. And you are getting up there on lift for the Edelbrock head valvesprings. And it should idle down with an auto around 750-800 rpm.