The best blocks, heads, and cranks can be found at a machine shop, already done. Machine shops are all the time getting stiffed on jobs. Find a very large busy machine shop and ask if they have any chevy blocks that they got stiffed on. That is how I got my 350 4 bolt. They wanted to sell me the block on top of paying the machine shop bill. I told them they could keep it, that they got the block for free, and I was more than willing to pay for the machine shop work they put into it if it was a good block. The manager saw my point and agreed. I let them freshen the block for me, and mic out the cylinders etc... so I could get parts and bearing for it, etc... also had them line bore, and chamfer the block, and mag it for cracks.

Sounds like the block you have is a real piece of work. Could end up easily spending the cost of a GM crate motor to rebuild it. The waxy sludge buildup is from running Penzoil or Quakerstate in an engine that run hot. These oils use a parafin base to control viscosity, and breakdown and seperate under higher temperatures. The waxy greasy sludge is this product. I've seen this on other heads, and in the lifter valley, so bad that the springs looked like they were setting in holes made for them.