ok that cam sounds ok but maybe the rocker ratio is too much .507/.510 lift.....is it just the exhaust lobes that are gone or intake as well ?...i would check the valve lift without a spring on there ,if its not coil bind then maybe it is bottom of spring retainer hitting on valve guide .it might just be that extra .010" lift that puts the damageing presure on the lobes ....i have preasures for a 272-h10 and they are 105lb closed 265 open but theres alot more too it than that (installed height and all that stuffs)sounds like your pretty on to it with the breckin procedure.seems strange that its the back cylinders with most wear(#8,7,6,5).i would look at simulating the lift on inlet and speaclly the exhaust without a spring in there and see wots going on .i have a set of gm 041 heads with a .467 exhaust lift and the spring retainer only just clears the guides with valve stem seals,depending on your installed height and rocker ratio this could be a problem . these are just suggestions and hope some else can shed some more light on the subject ....good luck ,get back to us soon
