ive run several flat tappet combos in the 425-450lb open range with no problems.

in fact, IIRC for one of the big flat tappet cams MP was selling i think they were rcommending the big triple spring.

however, i will say i dont feel the lifters of today are as hard as they used to be, and the engine oil doesnt have the amount of zinc it used to either....so what used to work fine, may not be the hot ticket any more.

i also feel the cam design plays a big role in how pressure you can get away with, as well as the rocker ratio.

EDIT- i checked my old MP engine manuals.
they show the recommended spring for the .620 and .655 flat tappet cams is the P3462887 triple spring, which has a rate of 558lbs/in.
157lbs @ 1.900 and 447lbs @ 1.400.

the "mild" MP dual spring(P2806077) has a rate of 530lbs/in(115 @ 1.900, 380 @ 1.400).