I finally broke the piston in the third cylinder loose. First I used a small piece of plywood, put it on top of the piston, and hammered it with a 10 pound mall.  Then, when that broke apart, I remembered that I had a 2 1/8 inch socket in the toolbox, which is just slightly smaller than the cylinder bore.  So I put it on the piston and hammered on it until I turned the crank enough to be able to remove the rod bearing caps from the 2nd and 3rd pistons.  Once that was done I tapped the pistons out of the cylinders.  Then I finally got the sleeve for the distributor drive out of the block by prying a screwdriver between it and the block, then removed the distibutor drive, which allowed me to remove the camshaft.  I was also able to pry the front main bearing cap out.  So now I just have to build a jig to remove the other 2 main bearing caps out and the block will be bare.

Randy said he's got the head all cleaned up and that he thinks there is a good chance that it's cracked.  Actually, I think that might be a good thing, because it would explain the problems in the 3rd cylinder. Now I'll have to decide if I want to have this head taken in and checked, or if I should take the one from the other MGB.  Randy said that one looked pretty good, and they are interchangable, so I might do that.