Do these horse power numbers seem somewhat surprising?

[IWillCookandEatChaka]

okay I have a factory spec'd (all either original or NOS HIPO parts...it cost me a pretty penny to get there too...lol) align bored, balanced and blue printed 1965 Ford 289 HIPO (factory rated at 271 hp). I have the shelby gt-350 engine improvements (holley 715 carb, shelby aluminum intake and a NOS set of hooker full headers, rather than the shelby Tri-Y headers), the only other modification is an msm electronic ignition (dist and coil) and the heads were ported and polished and of course the valve seats were 4 angle cut, and the intake manifold was also port matched to the heads. Shelby rated their improvements at 306 HP...I was assuming both the 271 ford rating and the 306 shelby rating were somewhat optimistic, so even with my improved ignition (over a dual point dist and 55 year old technology HP coil), full headers and a port and polish job...and of course a full blue print on the engine, I was expecting (or hoping for 310+, maybe 320 at best)...anyway after a break in period stretched over several pulls when we got the jetting and timing curves adjusted our first good pull made 325, with some more piddling and some more break in time we finally got a pull of 340 HP...I was shocked...out of a basically stock 289. Just curious if any other old ford engine nuts may be out there to shed some light or reactions.

fyi the car is a 65 HIPO mustang 2+2 (aka fastback)