2.6 vs 3.0 engine blocks
 

Is it possible to bore a 2.6L block to 3.0L specs without any trouble e.g. decreased wall thickness causing overheating or shortened engine life?

I thought both m103 blocks are identical, except 2.6L having thicker walls, however I saw that water passages are different and these two blocks don't look the same.

I doubt it. The two blocks have different coring. Both have the same stroke, but the 3.0 has a larger bore and they are siamezed - no coolant between the bores. The 2.6 has a full cooling jacket around the bores.

Heads are different with the 2.6 having smaller valves and ports. Too bad Mercedes didn't increase displacement by just varying stroke on the same long block like Chevies, but Mercedes never seems to do anything the easy way.

Duke

Heads are very different for sure, I just wanted to rebore the block and use the existing crank and rods.

AMG converted the M103 2.6 to 3.2L for installation in the W201, using the head from the 3.0L version for the larger valves.