Well, I have had both. I prefer an aluminum block for the weight reduction and the better cooling....Mine ran nice and cool on the street....Growth can be an issue. You do need to check the valves at full temp, no way around it. But once set, they were fine. And yes it has to be at full temp to really beat on it. We were not real happy with the Indy block we got. There was .011" of run out in the cylinder bores and the sleeves seemed real thin and it had other issues like the thrust face not fully machined and a few other things. We were not able to get all the cylinder bores super nice, but we found a bigger issue later. The bigger issue we had was oil pressure when hot. Ours always dropped off and we could not find it. Went thru a lot of trial and error, and could not find it. Ultimately we sold that block. Seems there may have been an issue with the last cam bearing machining and we never found it as the bearings come in the block already, but the guy that bought the block did....

The newer KB will be much better, but stay away from running the bore to max. Keep the sleeves as thick as you can. The 572 is pretty standard these days. I currently have a World Iron block. I am very happy with everything but the weight of it. And I will be upgrading my water pump this year. I have a very good cooling system, but its right on the edge with this 605 and an electric water pump...