That's pretty common to find crossbolt bosses in the 63-64 blocks. I've had C3AE-D 406 blocks so it's very possible yours can be bored out. You need to inspect the cyl wall cores to see. You need to knock out the blocks freeze plugs to access the wall core area. If a 13/64" drill bit shank is a snug fit good chance the block has 406 wall cores. Something common to the 406 blocks are "HP" cast in the block on the back, side and inside the block. You'll need to sonic map the block even if you do find thicker wall cores. It might be coreshifted and was only able to accept a 4.05 bore. But the drill bit test will confirm what cyl wall cores the block was cast with. That's pretty interesting as I've never seen or heard of a C3-D block being drilled for hyd lifters and having a 4.05" bore. So please keep me posted what you find. Every C3-D block I've come across has been a solid lifter 406 block with 4.13 bores with the HP markings cast in. I've had 5 of those D blocks and several other 406's as well. Can you send me some pics? This is a first that I know of.


G.