Having a standard 425HP engine and just bought me an B&M 420 blower!!
Can I use the GIL exhaust!??
5Psi will give approx 620 hp...what is max hp for the GIL system!?

No problem. I have used them with WAY more power than that. Just watch your cam choice, if you change it, if you have the lower risers. If you aren't going to change the cam, then no problem.
Eddie

The Gils will be fine. Max is about 600hp for an NA engine and probably more like 700hp for an SC'd engine before the start choking the engine.

my 510 in our race boat makes 715 i switched from my gils with dry tails to lighting with dry tails no gain in speed .
I might do a test at tyler crockets when i redyno it and do back to back test with gils-lighting-and my cmi to see how close they are.

Thank´s :-)