flamered

I have a stock '65 Skylark convertible--300-2V--which currently has single exhaust. I'd like to convert it to dual exhaust. I've been told that dual pipes should be smaller in diameter than the single exhaust (not sure at this point of the correct diameters--cannot find dimensions for them in svc manual), as larger diameter pipes with duals might somehow affect performance adversely--something to do with torque curves etc. Anyone know if this is true? Thanks.

Re my '65 Skylark convertible: I have conflicting information as to whether a '66 Skylark AM/FM radio will fit the dash of my '65--can anyone advise? I was hoping to simply switch the '65's AM for a monaural AM/FM. (I'm told the '67 is a different dash, but am unclear about the '66.) Thanks. P.S. Anyone have one for sale--a '66 AM/FM, providing it fits a '65?