the earlier 5.3L heads have 1.980'' intake valves which will hurt your flow numbers, if you can get them machined for 2.00'' valves you might have something. but for the price, you could find a set of P&P 241/243/799 heads and that would give you better numbers and thus, better potential for power. stay far away from the perimiter bolt(early style) 5.7L heads. if you want to make serious power, invest in a used L92 genIV top end with the higherflowing rectangular port heads and intake. they also come with much bigger stock valves than the cathedral port LS family top ends. this would be like bolting on a set of performance heads right out of the box. if you get the heads milled down and buy a set of shorter length pushrods, you can achieve a higher CR and make more power per stroke on the same fuel and air consumption