I know in regards to highperformance V8 blocks a large X in the block casting indicates a "HI-PO, High Nickel" block. However I've seen 300 six blocks with a similar mark on them near the casting #.

Ford cast the 300 at the cleaveland foundry. The same place as the351C, and 428 blocks.

Does the "X" block 300, have a raised nickel content? Is it a "HI-PO block?

If so I got a HI-PO 300

 

Interesting. Maybe you have a Hi-Po 240 Unless the 240 was also cast at the Cleveland foundry and has an X on the block. Maybe it just means that all those blocks are hard to break due to high nickel content. Who knows.

 

Well in a nutshell the X means High Nickle. A tougher more ductile iron results. Its just a bit tougher to cast and more expensive.
A HI-PO 240
it will go with my svt 144 six

 

Well, the truck I-6 motors (I'm going to guess the 240 is same material here, for sake of saying) are some pretty tough engines. I have yet to see one that was destroyed by anything other than catastrophic failure due to owner stupidity

You knew I had to go there. If I didn't, someone else might have

 

The "HI-PO" 300 had internal differences, mainly the forged crank. They were mainly found in the larger trucks (F-600), though a few found their way into the smaller trucks. With a 7-main bearing design, there is actually little reason to have a 'HI-PO block.' Add the stability & self-ballancing nature of the I6 & you could actually have a 'HI-PO' I6 with 4 mains (basically the earlier 262) with out too many problems. V-style engines are inherently rather unstable (there are some exeptions, I think that the V12 is one), which is why many of them are externally ballanced with the flywheel & harmonic ballancer. Also, you have 2 piston rods on each crank bearing surface, creating a lot of stress (hence the '4-bolt mains', cross-bolted mains, etc.).

This is not to say that the I6 design is without problems, try taking most stock inlines over about 6'000 rpms (if they'll even get there!) & there's a good chance you'll eventually get the crank to 'walk' right out of the block. I belive its called 'longitudinal tortional vibration' (?). It takes some work to get many I6s above the 5,000 rpm range (it has been done quite a few times with success) & usually its been the smaller, shorter-stroke engines. The ford 200, 240 & chevy 230/250 are good examples (though there a quite a few others, look overseas & there are a TON of examples).

I guess what I'm getting at (& probably a lot more long-winded than I intended to be - early morning, lots of coffee...) is that the "HI-PO" V8s were designed for HORSEPOWER while the "HI-PO" 300 (you could include other/earlier motors like the 262 or the chevy 292) was designed for TORQUE & durability. All the horsepower in the world sounds great till you're trying to get a loaded grain truck down the road, especially with an engine that will make its max torque around 5,000 rpms... In addition, put all that stress on an unstable, not-so-strong engine design & see how long a HI-PO 428 lasts (ever wonder why you don't see very many v6/v8s in trucks overseas...).
Edwin