According to the info both Intense and ZZP put out, the S1X and XP cams both max out the stock head clearance on stock 1.6 ratio rockers. Not sure where you are getting any of your info about adding rockers on top of a cam and stock heads Dan.
Research the stock head clearance and what you find is the math for the XP cam doesn't work on a stock head. It'* the addition of modded/LS style retainers and valve seals that give the XP enough clearance. The S1X is a touch smaller and does clear the stock seals with modded retainers.
Before going up to rockers on top of a cam or a cam that'* bigger than an S1X/XP you will need head work. When you have that done, you should automatically have the spring seats machined down so that you have a correct 1.800" of distance for your aftermarket springs as most of them expect a 1.800" installed height and you don't want to preload springs. Have the valve guides milled down for .600" or more of clearnace with valve seals installed and you'll be ready for most of the big boy cams.
The way to determine clearance for lift is to take the cam spec x rocker ratio.
00 Regal GS: All kinds of cool stuff.