The 80s something block has no threaded hole for clutch linkage and no dipstick tube hole for front sump oil pan(there are fixes for both of these problems though).These are only problems if your puttng it into an older ford.
<font size=-1>[ This Message was edited by: frdnut on 5/16/02 2:47am ]</font>
The early blocks are heavier, which means they can be stronger in certain area`s depending on which block it is and how old, which is why a lot of guys use the older blocks for strokers and high HP applications.
The 5.0 blocks are lighter. Ford wanted to save a few pounds so they took some material out of the engine blocks, not sure where they lightened them up, but I think are 25lbs or so lighter. "Thin wall" casting comes to mind, again not sure of specifics.