Mopar R1 block

Ok, I don't know who thinks a T/A block is better than an X is on crack.

Here is the pecking order

T/A
X
R

That's the order. For what you are doing an X would be fine. The X has a thicker deck than the T/A block, both will take 4 bolt caps. I had an X block go over 700 HP at 8000 plus RPM. Nothing wrong with that.

The R series blocks are a pain in the ***. I have one that I've nailed down to either being some bastard R1 or it's an R2. I know I have a folder with what I remember to be all the R block specs, but I can't find it. Again. So I've given the wife a standing order to find it, because I'm sure she moved it.

Essentially it comes down to this. The R series blocks all have 1 inch thick desks. Very important. They also have the front pan railed filled. Kinda a big deal. Some have magnum engine mount provisions. Some have magnum lifter valley's in them (mine has this).
Some are dry sump oiling only. Some are 59 degree lifter bank angle, and some are 48 degree. Some have Siamese bores so they can go as big as 4.220. Some can go only about 4.060 and some of the block come with a rough bore of about 3.885ish.

I've had several X blocks and every one of them sonic tested to go at least 4.125 but you have to sonic every bore on every block.

Some have 340 mains and some had 360 mains. Some had Ford mains. Some had 2 bolt caps. Some had 4 bolt caps. They will all take 4 bolt caps.

So there is some differences in them, and some commonalities.

If you can snatch up a new, unused X or R block I'd do it in a New York second.