Will MAC GT 3" Cat Back fit stock h-pipe?

Yes, it will. Mac supplies sleeves that slip into the end of the flow tubes to make them fit the stock H-pipe. That's a pretty serious cat back for most bolt on cars. Even a 2 ½” system will support 400hp. You may want to consider going with the 2 ½ " and just upgrading to the 3" tailpipes?