Copied from Ryan Johnson, Shady Dell Speed Shop website:

How do I identify a specific MP race block?

This is probably #2 FAQ I get asked on almost a daily basis. I'm going to start out with X blocks and work our way up through the generations.
X block is simple to identify, they have a large "X" cast into the passengers side front of the block to the left of the timing cover, behind where the alternator would sit on a factory engine setup. It is plain as day and you can not miss it. If you've got a block with a Large "X" here, it's an "X" block. Pretty simple. (NOTE: See Photo #1) MOST X blocks were 318/340 main, but a little known factoid is that a SMALL # of 360 main X blocks were produced. The lifter valley in an X block, closely resembles a factory production block. The majority of its strength was gained in the increased deck thickness and the main webbing/pan rail areas. The bores are generally also a bit thicker in all directions VS: a factory block. All "X" blocks are 59 degree lifter bank angle.

R block: Again, where you would see an "X" on an "X" block, these have a Large "R" cast in. (NOTE: See Photo #7) The "R" block was the successor to the "X" block. It was available in 59* and 48* lifter bank angles. The main benefit to the "R" block was the addition of availability of 48 degree heads, and the lifter valley area was greatly strengthened. All "R" blocks have 10 head bolt pattern. Some EARLY "R" blocks did not have the "R" designation, but yet had the traditional "R" series large lifter bosses, and 48 degree bank angle. These were generally pre-production blocks.

R1 block: These have a large "R1" logo on the passenger side front of the block. The biggest change in the R1 series was the addition of 18 head bolt bosses for use with the 48 degree large "W" series 7/8/9 heads. Both 59 degree and 48 degree versions available.
R1A block: Large "R1A" logo, again on the passenger side front of the block. (NOTE See Photo #4)These are 18 bolt blocks, with a cut out inner lifter valley that looks much like a factory block. (NOTE: See Photo #2) Both 59 degree and 48 degree versions available.
R2 block: Large "R2" logo on passenger side front of the block. "R2" still maintained a traditional wet sump lifter valley cutout. Available in 59* and 48* versions. "R2" are probably the rarest of all race blocks, they were only available for a short time.

R3 block: Large "R3" logo on the front passenger side of the block. Biggest change in "R3" blocks was the availability of Siamese bores, and the valley on an "R3" has been completely filled in with iron, to make running a dry sump application much easier on the engine builder. R3s were available in the widest range of configurations, including 360 mains, both 59* and 48* versions, and deck heights varying from 9.025" to 9.56" R3s are the most common of the "R" Series blocks that you will run into at swap meets etc.
Now that we have the basic generations covered, I'll go through to how to ID exactly what version of a particular generation you may have run into.

First off let's get the whole 48 Degree VS: 59 Degree thing out of the way. This is very easy to Identify, even for a novice. 59 degree lifter bores are always down close to the edge of the lifter valley. (NOTE: See Photo #6) That is a 59* R3 block. The 48* lifter bores are raised much higher on the lifter bosses and are much closer toward the cam tunnel. (NOTE See Photo # 2) That is a 48* R1A block. Also if you pay attention in Photo #2, I noted how the intake lifter bores are also moved side to side to accommodate better pushrod angles on the offset intake rockers. So it should be no problem to ID a 48* or 59* race block next time you see one for sale.

The next good way to try to ID a specific MP race block is the MP PART #. Now firstly let me say, that the #s cast into the sides of a MP race block mean NOTHING. They are CASTING #s. I know they all begin with the letter "P" which would lead you to believe it is a MP Part #, but that is incorrect. The majority of race blocks share the same casting #, so it does absolutely no good to know it. What you should be looking for is the 3 digit PART # stamping on the block if it has one. Look on the drivers side of the block, on the front, opposite to where you find the X, R, R1, R2 R3 etc logo. If there is a 3 digit # STAMPED into the block in large #s that is your last 3 digits of the MP PART #. For example if you have an R3 block and it has a 796 stamped in that area, that block is a P4876796 59* R3, with 360 mains, 4 bolt mains, and 9.56" deck height. Now, under the block logo on the passengers side, ie. Under the R or R1 or R3 logo, you may find a 3 digit # over there. What that is, is basically the blocks machining production #. Kind of like a VIN # for blocks. If you have 001 you have the first block of that version ever machined. 078 and you have #78 etc.

It's pretty obvious to identify 360 or 340 or Ford main journal sizes, so I won't go into that. Same goes with 4 bolt main caps, if you don't know what a 4 bolt main cap looks like you should take up pottery or knitting.

Now the last big ID challenge is deck height. This is a tough one to explain in words what to look for. But here are a couple things to keep in mind. If the block you are looking at has a blank oil filter boss (no provisions machined for an oil filter NOTE: See Photo #8) the block is 9.025" deck and meant for dry sump use. If it has a complete factory oil filter boss area, it is either 9.20" or 9.56" deck. So the oil filter boss area is a dead giveaway for a 9" deck engine. As far as identifying a 9.20" or 9.56" deck block, that is tougher. Examining the area of material between the deck and the front water passages is the best way to know easily. Most of the early blocks were not available in 9.20" decks, so this generally only applies to R3s.

And then we have the 340 "Resto" block. All a MP 340 "resto" block is, is a 59 degree tall deck, 340 main, 4 bolt R3 block that has had the extra 8 head bolt bosses per side milled off. (NOTE: See Photo #5) 340 restos come through machined at 9.59999" deck. (Factory blueprint spec is 9.600") Many "tall deck" race blocks will actually be machined to less than 9.60" which can make off the shelf piston/Rod combos troublesome. That is why they are called 9.56" deck blocks, they are meant to be run at 9.560" square deck or less.