In so much that nearly every Vega blew it's head gasket with the aluminum block/iron head configuration, I am not so sure that the addition of the Cosworth aluminum head would have add any less reliability from that stand point. LOL The standard engine wa s rough running SOG. I suspect that that contributed to its many problems.
When I was a kid I had a new Vega and was skeptical of any aluminum block/head combination coming out of Detroit because of the problem I had with the engine. It's not like I neglected the engine, I replaced the short block in my parents garage when I was 19. I knew what you needed to do to maintain a car. :~) A piston skirt broke on mine and that in turn punched a hole in the aluminum cylinder. The engine locked up pretty quick, having sucked coolant into the cylinder. I'm glad I was not on the freeway then it happened.
You are asking the wrong person. I made my living in the automotive business for many years but that was work. LOL. Does NASCAR require push rod engines or factory configured engines? Can they use any factory engine that they want? Yeah, the 5.7L engine is a DOHC design so there are 4 cam shafts, driven by a chains vs. belt/belts, thank goodness. Compared to most Japanese engines this one does not have any required scheduled parts replacement preventative naintainence. Yeah it can still have a timing chain go bad but I have to hope that the engine is a non-interference design. I can not easily see how that block could be configured to run with push rod heads, like you said, where would you put the cam.