I have a whimpy 265 hp 396 in my C20

[buckskins4ever]

I can only say that while your block numbers indicate that it SHOULD be a 265HP 396, that would be at the Flywheel and right at engine break in. If this engine has pretty healthy mileage on it, you could be realistic lower 200HP. Great for daily driver stuff but it would be best to pay for a dyno.........and do so after putting on a good set of headers. When doing a dyno run on the truck keep in mind anything happens to the engine is all on you....they will not be responsible if it blows. So grab your headers, check your compression, makes sure your oil pressure is good.
Good luck, the Dyno can help you.