That depends on who you believe. There have been rumors and even articles posted in excellence about casting differences between the blocks indicating that the Turbo blocks have more material but based on my personal measurements of the mains, cylinder walls, water jacket depth, and block thickness and they're identical. I also doubt Porsche would have spec'd different metallurgy for the blocks to increase strengths.

The early blocks NA are different from the later 44 and 51 blocks so you do not want to use that because the oil feed journals are smaller which is why the early cranks were x-drilled. Somewhere around late 84 and early 85 they opened them up. To be on the safe side (id you don't know what your looking at) stick with an 86+ block for turbo use.