Exhaust is a complicated issue and I am sure I will get flamed for this but here goes.
Many people believe that a more restrictive exhaust is better. I believe they are confusing back pressure with wave tuning. There are all sorts of pressure waves generated within the exhaust piping and sometimes a longer , smaller pipe works better versus 2.That is not always the case. Its the tuning of the pressure waves that delivers the benefit and a 258(I love those engines) can benefit with a single exhaust over 2 because of the low displacement.
However IMMHO all internal combustion engines generate there power from air/fuel. The more that flows thru (to a point) the engine the more torque and horsepower generated. I believe in as little restriction as possible from the breather to the exhaust tip.
End result of all this is you might get a little more torque by going back to a single exhaust but it's probably not going to be worth the dough to do so.