[doodad]

'64 GTOs came with a 389, not a 396.

But, I think the hybridization came from an SS engine or perhaps bore and stroke massage.

[Doohickey]

There was never a GTO SS (That was Chevy's performance package). Plus, the 350/389/400 were smallblocks; the 396 was a big block.

The smallblock 400 was introduced in the 1967 model year, which was a bored out 389. You could perhaps bore out a 400 an additional .030 over before the cylinder wall be came too thin in the area of the water jackets.