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soak it with penatrating oil for a couple of days...mine did the same thing and after 3 or 4 days it was begging to come out...you have to give the penitrating oil time to work...it will come loose...when you reinstall it coat it with anti freeze on the shaft where it contacts the block...
also, I still had to hammer on mine to get it out. There is a spot under the wide part of the dizzy where you can get a punch on it, and whack it with a hammer to try and get it to spin. You can also sometimes get a slide hammer on them and yank them that way.
I have used a small flat nail puller. You get it under the lip on the dist. and hit the puller with a hammer. If you can move the dist. just a little, then you can squirt PB Blaster into the crack. Then it should loosen up after a while.
I've used PB Blaster and it's OK, better than WD-40 but my all time favorite is Kroil from Kano labs. Stuff will unstick most anything, at least everything I've tried it on. Google it and you'll get to their website where you can order it online. Unfortunately I only discovered it after I ended up using an air chisel on the dist in my 400.
Unsticking the dist in a 351M/400 kinda reminds me of what Bob Uecker said when he was asked about the best way to catch a knuckle ball, "Wait until it stops rolling and then go pick it up." With the 351M/400 dist you can try to get it unstuck without busting it or just go buy a new/rebuilt one and not worry about tearing up the old one.