here's the GM centric shop that wins almost every road race it's cars are in amateur to pro.
http://www.phoenixperformance.net/
and these huge pcar shops...
Bill Rudtner at South Shore Performance has done some Z/28 development
http://www.rudtnersracinggroup.com/
Spencer at
http://www.speedsporttuning.net/ can make anything fast full race prep as well
Rick at
http://demanmotorsport.com/