Dyno Results

I have a 1969 Mach1 with a 351W, 4V, mild cam and headers.  It has a FMX with a shift kit and a 3.50 detroit tru track gear.  This car is suppose to be just under 300 HP from the factory without the added cam and headers.  I run strong,  I raced a 340 Challanger that was suppose to have a "375 HP crate motor" in it, and beat it in the 1/4 mile.  And also raced a Corvette and beat it out of the hole  and stayed a car length ahead until my car ran out of steam.  The Corvette had a much higher top end, I'm good to a little over 110, the Corvette is just getting going at 100.  
I just recently went to a Mustang Show and they had a Dyno setup in the lot.  I got talked into putting my car on the Dyno.    What a disappointment in the results.  I thought that I would exceed the 300 HP but my results were 189 HP with 270 Max torque at 4500 RPM.  
I didn't do the rebuild on the motor, the previous owner did with no documention.
Has anyone else done this and the Dyno was wrong?  Or do I need to yank this engine and start over.  I have a 1974 351 W block and crank that has never been bored sitting in the garage, its ready to built when and if I get around to it.

Give me your thoughts on this situation.
Mike  

I would say that results speak volumes.  It sounds like you have a strong and fun to drive car.  I think that's great despite the dyno test.  The Challenger must have dyno'd 150. If you are going to be serious about drag racing, then go all you can afford for what you expect, otherwise enjoy a cool, strong ride and realize that there's always someone who can beat you from light to light-maybe not tonite but tomorrow. There's a difference between the brake HP figures from "the day" and rear wheel HP on the dyno.  Many muscle car stars from 30-40 years ago will perform far fewer ponies on the dyno than what was advertised.  

Yep, the difference between "advertised engine HP" and "actual HP" (@ the rear wheels) is dramatic.  189/270 at the rears is pretty good for a daily driver.
jlg