The cams are larger in diameter because they have a 55mm bearing journal (2.165inch) vs the SBC Journal of 1.868", on top of that the lobes have a relatively low lift for the valve lift (because of the rocker arm ratio) so that makes the cam stiffer as well. The stiffer a cam, the less it twists torsionally so the valve timing is more accurate. If you take valvespring pressure away this also helps lessen the effect of the twisting moton the valvesprings put on the cam, but then again so does the acceleration and velocities of the lobe. Either way the BEST way to fix the problem is to make the physical journal diameter of the cam larger so you have more meat around the base circle, which means a stiffer cam.

Bret