The Hughes 1110 single spring works fine with the voodoo 268. lunati told me if it has around 120psi seat and close to 300 psi open it should work regardless of it being dual or single springs.
i have a cyl. heads with dual spring work, but was so hard... then the engine maker took away the inner spring and let only de outer spring... they work fine with the comp cams 274 ... and now With the Crower 267