I agree with Techinspector1. I think your cylinder pressure was pretty high with the short duration cam and you've been detonating the engine. I just calculated your compression ratio as follows:
4.155 Bore
3.75 stroke
72 cc combustion chamber
0.039'' head gasket
0.010 deck
-5 cc valve relief
10.457:1 Compression ratio. Thats pretty high with iron heads and pump gas, especially with the small cam.

If you go to a 0.060 overbore I'd recommend you fill the block with block filler up to the bottom of the freeze plugs so you will have enough strength in the block for that overbore size.

Suggestion: Cam the engine just like it was a big block Chevy. Most of the cam info for SB Chevy's are for the 350. The 400 type will handle a lot more cam than the 350 will. A good choice for a street 406 would be something in the 230 degrees @ 0.050". Maybe 235-240 degrees on the exhaust side. You can drive it every day and it will still make good power.

Have fun