Josh, you are going about this the wrong way. I would never cam an idi without pulling the heads and checking your piston to valve clearance. I believe Russ requires comp 910s to be installed in combination with the cam. You run a high risk of ruining a “healthy” engine as you say, but leaking headgaskets are a sign of an engine that’s been run hard, and overheated as mentioned previously. If you are set on putting a cam in it, then pull the engine and do it right. But I think you will find more of a performance improvement by adding an intercooler or a ww2.
I think you might be able to check valve-to-piston clearance with a feeler gage if you wanted to - using a solid lifter(no hydraulics to compress on you) and adjustable rod, adjust to 0 lash... then put a feeler gage between the valve stem and rocker. crank the engine over by hand and you ought to feel when the valve hits the top of the piston(with, say, a .030 shim under it). As long as you are moving the engine by hand, you shouldn't have issues with damage or anything...
Just an idea...
As far as physically swapping the cam with the engine in the vehicle, yes it's possible - with valve covers, intake, valley pan, IP, front cover removed. Oh, and you'll have to cut out the vertical support in the core support that's right at the front in the middle. Do that and with the grill and everything in the way removed, you can do it.