Sounds like ya did everything right ?????
I would suggest using the Valvoline VV851 20W50 , 2 cans of G.M. E05 and leave the inner valve springs off untill you have done your cam breakin for 30 minutes @ 2500 rpm,then go ahead and put the springs back on and run 500 miles before you change the oil. You can then go back to your regular valvoline 20w50 racing oil as that VV851 is a little pricey..... You didn't use the old lifters did ya ??? If you did thats probally why it failed...