Isky 6005 valve Spring Rate is 275 lb/inch. Isky recommends these springs for all of their Y-block cams. Ted has said that he's not had a problem using these springs with any of the Isky cams. 

I intend to use PAC-1286 beehive valve springs in my G heads/w stock-size valves. The PAC-1286 spring rate is 283 lb/inch. The recommended installed height is 1.75. My net valve lift will be 0.485. The open pressure/w recommended installed height will be 262. I am considering shimming the springs to increase open pressure at .485 to 277. This will increase the seat pressure 15 pounds to 140. Considering the Isky springs have a seat pressure of 135 pounds at the recommended installed height, am I on the right track by shimming the PAC springs for the increased open pressure? In order to prevent premature wear of cam lobes and lifters, Ted recommended keeping open pressure below 300 lbs.

Keep in mind that the stock size Mummert stainless valves have undercut stems and are lighter than stock OEM valves. The PAC beehive springs and retainers are lighter than stock components as well.