Without knowing your cam specs, I think 300ish RWHP is about right, that's about 380hp. 440's especially with stock heads are just not big HP makers. They don't make good dynamic compression (comparatively to other rod stroke combos) and the factory heads can't breathe.

I had a strong running 440 (.030 over) with flat tops, a Hughes solid flat tappet cam and Edelbrock performer heads with 1 7/8" headers. The engine made 360 RWHP (or 450HP). I also was disappointed but the 3700lb B body ran 11.90s @ 115mph.

Bottom line: 440s make a ton of torque and that's why they were feared on the street, they got things moving in a hurry.