Multiple misfire codes ona a 360?

I think these codes are all related to a single issue. The P0455 is a "large" evap leak which will cause the inaccurate O2 sensor readings (cats and fuel trim) as well as the multiple misfires.

Searching for a vacuum leak appears to be the place to start.

 

An evap leak is not the same as a vacuum leak. A decent tester will tell you which code actually turned the light on. Many of these codes, particularly missfire codes, can be set without a check engine light. I suspect the fuel trim or catalyst code has turned on the light. Straight pipes will result in a check engine light and catalyst efficiency codes.
Pulling up the freeze frame data will also tell you the codes and the basic operating parameters when the code was stored.

 

P1124 is one you should be worried about, it telling you that it's too lean.
run too lean and you'll burn an exh. valve. that's big $$$ to fix.

you're running test pipes, what are you doing about the post-cat O2 signal? without cats the ECU sees a rich condition then codes a non functioning cat and pulls the fuel trim, but if it's too lean you'll get detonation and burned valves. unless you have O2 simulators fooling the ECU into 'seeing' cats the car is going to throw lots of codes.