HOw much valve lift is safely possible without hitting the piston on a stock L98 Engine?

 

WELL , THE MAIN PROBLEM THAT YOU MAY HAVE IS COIL BIND. BEFOR I GOT MY NEW HEADS JUST TO SEE WHAT WOULD HAPPEN I BOLTED UP A SET OF 305 HEADS. THE CAM COMPLETLY COMPRESSED THE SPRING.
dONT KNOWIF THIS HELPED YOU BUT THATS WHAT HAPPENED TO ME

 

don't know exact number but it's at least .600, and as speedking said, you'll get coil bind with that much lift on stock springs. if you want to run a high lift cam, you'll need new springs that won't float or bind as well as screwed in studs so they don't pop out.