I would agree with the last few posts, as the significant changes were made in '80. They also changed the dipstick location to the side, deleted the clutch pivot boss, and slightly changed the distributor shaft diameter IIRC. In-spite of all that, I have never seen any true (or at least noticeable) difference in strength, and most info seems to be Urban Legend. Ford rates the later blocks at 7000 rpm and 400 hp, though (with a good tune) they seem reliable to 450-500 if the rev's are kept down, or 550-600 with lower rev's and boost. They can often go higher, but it's a crap shoot and all in the hands of the tuner if built well.
The primary failure point is the caps, as they pull away from the block registers and crack the block at the main cap bolt holes, often splitting it in half. Surprisingly, I have not seen that the heavy K-code and Mexican caps reduce this problem substantially, though I would hope it helps somewhat. Girdles (due to geometry) are considered useless, but hearsay indicates keying the caps is useful. This means balancing is critical for high numbers, along with good tune and a compromise between rpms and HP. Like any block limitation, they can take high revs or high power - but not a lot of both. If you're going into that neighborhood, I would consider aftermarket. Thats my 2 cents.
David
