225 to 235

I have a magazine form the 60s that says they bored a slant six .130 over, so .080 would not be out of the question. There is a lot of metal between the cylinder wall and water jackets, which is also one of the reasons they last so long. Mine is bored .040 over which works out to just under 230 cid. I plan to go bigger next rebuild.

Bore 3.40 + .040 = 3.44 formula is pi x radius squared x stroke x no. cyls

so 1.72 x 1.72 x 3.14 x 4.125 x 6 = 229.9; or in your case with a bored of 3.48; 1.74 x1.74 x 3.14 x 4.125 x 6 = 235.28 cid