1978 Chevy Corvette lack of power

I am the original owner of this 78 Vette.

I changed the plugs and wires and found that the drivers side bank had clean plugs. The passenger side had the head of the plugs almost gone. The drivers side exhaust (no crossover pipe) had much less exhaust output than the passenger side exhaust. I had the pipes behind the dual catalytic converters replaced including the mufflers and they told me the cats were both good. Power 0-60 used to be around 7.2, now it is around 9.5.
Help!
Kim Thaler