[olddog]

The 390 cid is a 4.050 bore by 3.78 stroke (in inches)
The 428 cid is a 4.130 bore by 3.98 stroke (in inches)

Many of the 390 blocks had cylinder walls thick enough that they could be bored out to match the 428 bore. Many others were not thick enough. Even when a 390 block has enough meat to take the 0.080" over bore, it is possible that there was a core shift in the mold when the block was cast. In these cases, one side of one cylinder may get too thin. This is why it is recommended that you pay the money to have the block checked before you have it bored, because if the 8th cylinder breaks into a water jacket, the block is junk and you still owe the man for boring the block.

So in short yes a 390 block can be bored to the 428 bore and result in a good solid engine, but not all 390 blocks can be bored that far. What is important is after the block is bored, how thick are the cylinder walls? If they are too thin, it will have overheating problems. I would not trust that all shops know what they are doing. In fact, I'm certain that there are people boring too thin and gambling that it will not give you trouble.