Danny,
Good find!
Two things.......
1) check the bore.....see what you have to work with. If it is still standard dimension (4.0"?) then you are ahead of the game. I believe that these blocks were cast with more meat.....but couldn't tell you where the metal ended up.....sonic testing will reveal wall thicknesses. Never jump to a .030" overbore......see if +.010 will work, if not then .020"...... Custom pistons are available from........ CRS...... Cleveland expert here in the US......likely from guys down under as well!
2) Can't tell from your pictures, but look at the block castings at the main bearing caps, from the side. If there are two bumps where the extra main cap bolts would go, then you have the "improved generic block casting" which can be machined either way....... Likely you do, this being a 1977 produced block. In the US, the blocks built in 1972 and later have the improved block also, although perhaps less improved than the XE version......
3) Don't discount this block just because it's a two bolt main. Unless you are going to try and spin this at 8K rpms for more than 1/4 mile at a time, you really don't need the extra bolts........ Also, your main caps actually may already have more meat on them! I can't say for sure, but they look to be pretty beefy!!! If this is the case.....again, you are ahead of the game.
Be a realist about how you are going to use the car...... most never see 5K much less 6K after which the connecting rods become the most likely to exit the engine..........
OK, that was three.......
Good find!