Thanks for your response. If there were no improvements, in material or design, made between 68 and 77 in the manufacturing of the blocks, I will undoubtedly disassemble the block not in the running vehicle (the 68 block) and verify it is a good candidate for the rebuild.
It is the simplest route since I get to continue to drive the vehicle whilst building the new engine.
BTW, an example of a difference in design and manufacturing. 68 230/250 heads ok...77 integral heads not so much. But that example is easily identified.