261 blocks can be a little funky in the overbore bore department. Between the Siamese set up of the cylinders, the use that these motors were put, and the years of rust deterioration on the outside of the cylinder in the coolant jacket, going .080 may lead to more trouble then that block is worth. Generally, the 54-55 blocks, the so called first series, are the thickest and can take pretty big over bores - sometimes as much as an 1/8. The later 55 through 62 blocks, and the service replacement blocks, are all over the place. I have a 261 block that showed a rust through spot at .030. that block was a service replacement block and when I sonic checked it, yep, every cylinder was thin, too thin to go any further. Other of the later 261 blocks were fine to .060. So I think between just the bore taper from use, and the rust issues emanating from the inside of the block/cylinder, going further then .060 should really entail a sonic check no matter what series block is involved, and even then its no guarantee that a spot wont appear after the boring bar does its work even if the sonic examination suggests the cylinder can take an .080 bore.
So I might consider a sonic check as a first step before you start sinking $$ into this particular block.
