KE has this kit with their own programming and just curious what numbers it might make? Also can the stock heads be used?

http://www.keitheickert.com/detail.aspx?ID=9082

Part#755-6301K
Degrees Duration Intake: 214
Degrees Advertised @.050 Exhaust: 226
Lift with 1.7 Rocker Arms Intake: 553
Lift with 1.7 Rocker Arms Exhaust: 587
Lobe Separation Angle: 112

That kit and cam is for a carburated HP500 engine and will not work properly in your 502mpi EFI engine. Bob Madera at RMBuilders can give you good cam advice.
Check with some of the other builders here on OSO, but my expierence is that the 502mpi 's tend to top out around 450HP with whatever efi cam and ECM mods due to the restrictive intake system and fairly weak GM iron heads. You should try and spec and use a complete system approach to modifying a 502MPI to get the best results and not just do parts changes and cheap fixxes.

Best Regards,
Ray @ Raylar

Ray pretty much nailed it. If you want the biggest gains from that engine, then replace the intake. I use an intake and throttle body from Holley. It is a really nice setup that will work with your ecm and harness. The intake setup alone may only be worth about 20 hp, but it is essential for any substantial upgrades. It will support about any hp level you can attain with your current engine. I have used them up to 800 hp. With the intake, cam and a bump in compression, you could easily make 550hp. Here are a few pics of the intake. This engine started as a 502 mag, just like yours. Give me a shout if I can help in any way.
Eddie