will cast iron heads replace aluminum heads on a 200 astro V6 vortec?

The 2000 Vortec V-6 ("W" series engine) is factory assembled with cast iron heads.

2000 Chevy Astro van. After replacing head gaskets, having heads shaved, valves redone, replaces radiator and changing oil 3 times, etc. etc. i drove it to the gas station for gas. I barely made it back. Water in oil again. New gaskets, Both heads were torqued 3 times each, stepping up the torque in sequence. i watched them. Now what?

What is the condition of the lower intake flanges?

the block must've been cracked. Heads were checked again and good, pulled it out. I wanted to ask how you get to the motor mount nut that's up inside access hole on pass side. I can only see one and my swivel won't quite get it. Drivers side i rem/repl ok