The factory reliefs on Ford flatheads was done to reduce the thickness of the metal in this area and reduce the tendency to crack the block, due to uneven expansion. Any extra flow was more than offset by the reduction in C.R.
Rounding the corner at the top of the bore will gain some flow, but the extra hassle involved when re-ringing the engine makes it questionable.
Most flatheads work better with aftermarket high comp. heads, but a Mopar 6 can be planed without any problems.