I noticed that the original cams have lifts around .425" or .430" in the later years. What is don't know is the answer to what you asked: the stock piston to valve clearance. I can only guess that is is over .100" as the standard was bigger back at that time, plus this was for production, where things are even more conservative.
But, the ramps are likely faster, even on that Magnum grind, than the older stock grinds, and so that points to an upgrade in valve springs and that cost.
I personally would not pay a lick of attention to what Summit or Jegs says... they are sellers, warehousers, and shippers.
The one consistent warning for the 273 is on the cam size.... don't go too big. If you drop the cubes, then any loss of torque with too big a cam is gonna be a lot worse.
What year of 273 do you have? And trans? And have you looked at the head casting numbers? I am curious of you have the closed chamber ones; sometimes they get swapped due to age and you dunno what you really have. I ask because if the open chambers got in there, then the CR is down and you gotta be even more careful with too much duration.
It may pay you to look at other threads and cams discussed for the 273, you are making several steps up in duration and that is gonna move the RPM band up maybe 1200-1500 RPM. I also depends on how you plan to use the car.....cruiser, driver, or racer....