A thin paint scraper works great, if you do end up scratching either of the mating surfaces on the cover or block a few strokes with a flat single cut file will clean it up.

Sometimes the corrosion around the broken bolts is the issue. Heat can be your friend as the aluminjm expands faster than the steel bolt

So I experienced this same problem recently. On my 240 motor there were thirteen on the cover and seven on the water pump plus the pan. The bolts that cause you grief are usually the ones around the water pump the broke off. Rust has filled the gap. After I reverified I had all of the bolts out I used a rubber mallet and PB Baster until I created a gap to get a gasket scraper worked behind the cover to help put pressure on it while tapping on the sides with the mallet. Everytime I started to give up I sprayed more Blaster in and took a long break. Eventually it will start to move and that is when you won the battle. PB Blaster love help from the heat if you can.

Just keep at it slow and steady. (I have no patients so avoiding a bigger hammer or more pressure was hard for me to avoid. )

Got the timing cover off today.  Driving in the double ended sheet rock knife blades all the way around, followed by the gasket scraper gave me enough gap that I could whack the distributor housing and lower radiator housings with a 4# hammer plus a wood 1"x2" block alternately and slowly walk it off.  Whew!

The three sheared water pump bolts were all proud of the block by 10mm+ so used some pen oil, hammer shock, MAP cylinder then vice grips slowly back and forth, until they all yielded.  Whew!!

Although the timing cover came OFF w/o destroying the head gasket or oil pan gasket, not confident about reinstalling the cover w/o screwing up one or both of them.  Maybe should remove the rest of the oil pan bolts, lowering it and moving it out of the way slightly.  Then install the timing cover with the oil pan last?

Dropping the pan makes the gasket suspect. If you lower it you might as well take it out. Once it is down all the way rotate the crank so the pan will clear number one cylinder crank counter weight.  I cleaned up the pan gave it a new coat of paint used gasket sealer to attach the gasket and waited over night to install it.

are you going to do the timing chain?

Need to leave Friday morning to participate in the Snow Ball rally, so only have two days to get everything back together and running. 

Wasn't planning on doing anything with the timing chain, but if there's something that needs checking before I button things up, I'd like to know what to check.  Same goes for the distributor, oil pump, starter, radiator, valve cover before button up.  Thanks!

Ken,

Im over in the EastBay, lmk if you need a hand (or an eye)