Oh sure you can pull it apart without taking the engine out of the chassis.
The problem is you cannot put it back together with the engine in the chassis.
AD
Let me guess.
You can't get the timing chains back on and correctly synchronized?
It's A LOT easier to work on an engine on a stand than inside a big steel box with lots of stuff in the way.
There are some tricks to repairing a spark plug without pulling the head, but see the previous sentence. If you crossthreaded a spark plug, you're going to have that much more trouble with an insert. Things get REAL fun when you break a tap in there.
Any chance this is #3 and you have air conditioning? If so, you'll NEVER fix this thing with both the engine and the evaporator installed.