Normally .001-.002 of material is removed when the block is align bored. Offhand i can't remember the exact measurement of the mainsaddle bores. Both 406's i had used a grooved crank on the mains and were C2 and C3 castings. But it's hard to tell if that's the original crank. It may or may not be. You might be able to tell from the timing set as align boring brings the cranks centerline a little closer to the cam. In some cases a shorter timing chain is used. You can inspect the main area and see if it's looks like it's been machined. Original blocks would have a patina on the main face area. Best thing is maybe bring the block to a good shop and have them measured. I'm sure they can tell. It's hard to say otherwise without seeing the block in person. Either way it's a really nice block to have. What heads are on it?

G.