i have a 99 z28 M6. i plan on adding longtube headers and a cam this spring. i already have an air lid and magnaflow catback. what size cam do you recommend? i was told 224/228 585 or so on a 112 +4. i am shooting for 400 rwhp. also which vendor have you guys had success with?

 

You'll need more cam than that to make 400 rwhp.
And you'll need way more parts than just a cam.
You'll also need:
valve springs
pushrods
gaskets
and you should change:
oil pump
timing chain

 

Agreed. Check out the Futral Motorsports F13 cam. Tons of guys have had luck with that cam and I've had luck with two of their cams.