I don't know about the gas engines for the chevy, but Toyota has been using aluminum blocks for many years in their trucks. I had an 85 4x4 1/4 ton Toyota and it had an aluminum block, ran great, no problems, and everyone likes these trucks for their durability. GM's warranty now is good so whether that means they believe in the quality of their product or they over-charge enough to be able to replace it before the warranty expires, I don't know.

 

I'd say get the aluminum block... Or the iron block... it doesn't really matter...