More drop in Beehive info for Vortec 4.3/305/350 for extra lift.
I'd rather use a Beehive than the 981. The Beehives smaller retainer is lighter, and acts like a lighter retainer without the cost. The Beehive springs arent the cheapest although teh 2002-2004 LS6 springs are pretty cheap.
Theres a ton of info in this thread along with pics and caliper measurements of how much clearance you can get.
Looks like about 0.560" lift with stock L31 heads with 787-16 retainers and 638 locks.