You really need to pull the crank and rods to notch the block. I suppose you could try and catch all the shavings, but I wouldn't do it on my engine. It's kind of like porting the head with the head still bolted to the block. BAD IDEA. So disassemble the short block first. Then you can start to notch the block.
What you want to do is get an L28 gasket, lay it on the block with the dowels in the block. Use a pick or a scribe to scratch a line in the block that represents where the headgasket ends. Use a die grinder with a carbide burr and you grind away the metal so that you push the edge back to the line that you scribed. Don't go too far into the cylinder, because you don't want to affect the top piston ring. I would not go more than 2/3 of the way down to the top ring. You'll be able to tell where that is because the top of the cylinder will be discolored above where the rings stop. That's pretty much it.
Oh then remember to have results like Norm you'll need to shave the head .070 and shim the cam towers up, then redo the lash pads. All more labor intensive stuff.
Or you could buy an L24 head. Matman has about 10 heads, I'm sure he'd sell you an E88 or an E31 for a reasonable price. You might want to PM him or email him before you get into all this other BS.