A siamese block has cylinders that are joined at the sides.
A non siamesed block has a water jacket surrounding 360 degrees of the bore.
The siamesed block can be bored out larger because of the extra cylinder thickness. The cooling issues of siamesed blocks seem to be caused mostly by air pockets, so a properly designed cooling system with all of the right passages opened up takes care of it.
The main webs that connect the two halves of the block are what keeps the block from splitting.
The late model 5.0's were cast with thin main webs. The Ford sportsman block, mexican block and K-code hipo blocks are all stronger due to the fact that they have thicker main webs and they all have two bolt mains.
The 4 bolt mains probably make the block a bit stronger, but if you need 4 bolts, you have one very healthy engine.
The blocks with 4 bolt mains are definitely stronger than a sportsman or mexican block, but mostly from the fact that they just have a lot more iron in them.
I'm going with a Dart sportsman. It has two bolt front and rear caps with 4 bolt caps on the middle three. This makes it easier to find an oil pan that will fit. It also has very thick siamesed cylinders that can be bored to 4.185", or more if it checks out thick enough. It is based on the same casting as that 2,000 hp dart, it's only weakness is it's front and rear caps, so it's probably good for 1,750hp.
I don't need 1,750 horse power potential, but I'm going to start with a 4.125" bore, so the dart is worth it just to have the extra material there, the next best thing is the Ford sportsman which is no where near as strong, and the maximum recomended bore is 4.030".