I can see it being possible theoretically if you removed all of the head studs, but that presents a few problems.
1. Steel head studs in an aluminum block = galvanic corrosion. It's been over 40 years since those studs got installed and convincing them to leave may be impossible without a lot of heat, which you can't apply to the aluminum case threads if the cylinders are on and in the way. I also don't know this for certain but I imagine the factory would've used threadlocker to install those studs and that requires >350 degrees F of heat to break.
2. You would have to line up the cylinder base and head while reinstalling and then install the studs through them, as when the studs are already installed the head is impossible to put on. Not enough clearance between the top of the studs and the frame.
3. Keeping the gaskets aligned while you try to install the studs between the aligned cylinder base and head. I suppose you could achieve this by using Shellac or gasket dressing to glue the cylinder base gasket and head gasket in place during installation.
Just pull the motor. Seriously, it will easier than what you're trying to do. I've done it twice and while I wouldn't call it easy the official procedure is very doable with 2 people and a floor jack.
You can even cheat by laying the bike down on its side on a pallet or other stack of wood supporting the engine with cut sections of 2x4, unbolting it from the frame, and then having a buddy hold it in place while you raise the bike back up to vertical. As long as the engine is drained of oil and the fuel tank and battery are elsewhere, and the front brake master cylinder cap isn't off, leaning the bike all the way over like that onto something wooden should cause no problems.
