Old FH race cranks ( before the 8BA )

[3W Hank]

A 3.7/16" bore ( MAX ) and a 3.785" stroke is over 280 cu in.
A 3.75" stroke ( same bore ) is 278 cu in )
Not much gain.

A classic Merc 4" and + 1/8" bore is 278 cu in.
3.75" stroke and 1/8" bore is 258 cu in.

My 41 truck-Merc/rised intake block is told to has ca 0.25" wall now ( sonic tested in Ca ) and bore is 3.3125" or 1/8" over std.
If I would 'dare' go 3.4375" I has wall that get thinner by 0.06".