The only times I've had antifreeze "attack" a gasket is when the gasket is so old and crumbly that it would be virtually useless as a gasket anyway. I've done at least 50 engines with this method.

A lot of cars have o-ring thermostat gaskets now...and in the 4 years I've been doing this the only sealing problem I had was when I did an old Torpedo silver head .19 and the black head gasket material partially disolved.(mainly because it's 40+ yrs. old )

If you're boiling it you're getting it WAY too hot!
Get a cooking thermometer...shouldn't need to get over 130-140 degrees.

Best way to do it is to make a basket out of steel window screen and a piece of music wire loop handle, so you can suspend the engine in the basket...
Use another piece of wire straddling the pot, and hang the basket, so the baskets bottom is raised a little above the bottom of the pot, and all the crud can come off the engine and fall thru the holes in the screen and settle on the bottom of the pot.

Also, if you dis-assemble the engine, you won't have to fish around in the cruddy bottom of the pot for the parts

Edit: Another thing about the need to dis-assemble an engine...
I finished cleaning an O.S. .91 Surpass for a friend a few days ago, and didn't remove anything except the muffler, which I cooked along side the engine.
My buddy flew the engine yesterday, and no problems whatsoever...runs like a champ

Just make sure you flush all the liquids out before you start it, so you don't "hydraulic" it...

Only thing you might have to do is cook a muffler, or head for a long time because sometimes the caked on stuff can be real stubborn. That's when a variety of "detailing" brushes come in handy...(they sell sets of detailing brushes for detailing cars )
The one with soft brass bristles can help with a really browned muffler.