Okay, basically every time your intake valve closes, a "shockwave" travels up through the runner. That wave moves at the speed of sound since it's just air in there. The trick is to have the wave in effect "bounce in time" with the intake pulse. Remember the 4V Cobra intakes with the vacuum-motor-operated intake shutters? Heck, BMW has an intake that changes length all the time (it supposedly acts like a telescopic antenna does, except in a circular fashion), computer controlled of course. Catch is with cast manifolds, when you calculate runner length, it's only good for one (or two if it's long enough) RPMs, say 2000 and 4000 (double the pulses). I don't have the exact formula for it, but I'm sure Carrol Shelby has a book out containing the formula. Not too hard, but definitely worth considering when planning an engine combo.....unless you're going with a blower. Then all that gets tossed out the window because pressure and all that jazz factors in.
Back to your camshaft, which you say you want long duration. That means your power band is going to be higher than what the stock Mustang's is, right (266/266)? Okay, again look at the Performer and Victor manifolds and their respective RPM ranges.
Notice how short the Victor's runners are? You can change valve cover gaskets without removing the upper intake! Also notice how much low end a stock Mustang has? Same reason: the intake is tuned to the engine's operating characteristics, that being low-RPM torque (small exhaust ports, 55mm TB, smaller valves, long runner intake). This torque comes from keeping the port velocity high. Kinda like running the same volume of water through two different size hoses. Okay, kinda got off subject there. If the runners are shorter, it will accomodate higher RPMs because the sound pulse will be of higher frequency as well, giving you that "ram" effect I was describing earlier. Hehe, I'm jumping all over, aren't I? Sorry....just that's the way I visualize what's going on. Think of it like this: visualize you're standing at the end of a hallway of X length, and you throw a tennis ball down toward the other end. This is like the intake valve shutting. Now, you want that ball to come back and hit the door behind you at the moment somebody opens that door behind you (this is anticipation of the next intake event) and smack them in the head (representing the next air charge rushing into the intake port). If you can make that hallway(or the runner) just long enough, it can happen. Remember though that you cannot alter the speed which you throw that ball. Does this make any sense, or is it just me because it's 4:52am? Hope this helps you out.
