I owned a GT500 which has the same heads as does the GT motor ..... those Im pretty sure where a 4 valve design.
I havent heard anything about plug issues .... There are a couple of really big companys that specialize in the GT .... HP perf i think is the name of one company
HP Performance - Home
Tony Gonyon
Owner
329 Park Ridge Ave, Suite 6
Orange Park
Florida
USA
32065
904 276 7223
Fax
Tony did Joe's GT .... which is widely known as one of the fastest GTs on the planet ... if there are any issues with this car he will know
Hope this helps