mellings pushrod for magnum heads?

0.060" is generally accepted as the maximum preload for a hydraulic lifter. If you had 0.015 with a 7.625" pushrod and went to a 7.700" that is 0.090" of preload, 0.030 more than the max.

You are likely to have pump up issues at higher rpms resulting in valves being held off the seat.

The 7.650" pushrods from Summit seem like the way to go.