You will have a hard time finding an X block. They are pretty rare. I'd think you'll have a better shot finding an R block.
The next step down is a T/A block. Also likely impossible to find. If you do find one, you will pay through the nose for it, probably more than even an X block. I bought a T/A block in 1985 and used it until 2000. I bought two X blocks and eventually sold the T/A block to a guy doing a knock off T/A car. He paid through the nose IMO but said I gave him a great deal.
My point is the older it is, the harder it will be to find. I've checked several X blocks by sonic testing and all of them would go at least 4.125 bore but don't buy anything over 4.04 without having it sonic tested.
As for th Pcar stuff there is zero difference in metallurgy or anything else. They are the same. I won't ever use a 4 bolt cap on a Pcar block. It is actually weaker than a 2 bolt with studs. Use Hard Block just over the top of the core plugs, stud the mains, get your clearances correct and go.
Hope you find what your looking for.
BTW, a friend of mine found the X block I now have about 18 months ago. It was BNITB with the paper work. I only paid $800.00 USFRN's for it and I got it because the guy selling it wanted to make sure I wasn't going to flip it on him and make money. He wanted it used. The stupid thing was people were trying to beat him down on price. I drove 3.5 hours one way with folding money for th guy and was damn glad to do it.
