This billet aluminum water neck is only $17 for a Chevy and I've read that the Chevy ones are the same bolt pattern as Mopar. The thermostat might be different but those are cheap.

Billet aluminum won't warp, is that correct?

http://www.summitracing.com/parts/spe-4931/overview/make/chevrolet

1970 Challenger, all aluminum 528 Hemi, HDK suspension, Tremec 5 speed manual