Does 280lbs @ .512 sounds about right for hydraulic flat tappet?
125lbs @ 0 lift

 

For a single spring it would be at the very limits of the spring. If it's a dual spring (not counting the inner damper) it should be fine.

For lift a little under .500", Comp Cam recommends their 981 spring. For lifts over .500", they recommend their 986 spring. Their 986 springs are good up to .540"

 

yea I was looking at the 986 but naturally pressures vary depending on assembled height