I found out that 360's and 318's were cast in the same mould, so the block will say 318, but look for a X on the driver's side and it's a 360.
The 318/340 main journals are different size than than a 360, so the cranks won't interchange, which is what makes a 360 a 360 anyway (longer stroke). Sounds like he was talking about this.
http://www.moparaction.com/Tech/questions/BIGGER_SMALL_BLOCK.htmlParaphrased from Small Block A" Engines";
The "X" block was introduced in 1979 as the Moper Performance Parts "X-Block". It was a race-only block that came in both 2-bolt and 4-bolt main versions with a large "X" cast on the front passenger side. It was delivered with a 3.91" bore (318 size), and could be machined all the way to 4.06" (or 340 +0.020"
. It was extremely stout and heavily reinforced with a higher nickel content that made it 25lbs heavier than a production block. It was eventually replaced by the "R" block, and the later "R3" block.
As for the Dana 60s (originally TRUCK rears) - Ma Mopar was the ONLY manufacturer that put them in production cars, which is a true testament to the power that they were capable of producing. They must have known the limits of the 8.75" since they started using them, but the 8.75" is still good for the street and less than extreme track duty. - RR
Edit - Below is a pic of an X-block posted by Scamp 408 on the A-body board. - RR