mufflers or no mufflers??

I have a 66 valiant that has a rebuilt 360 motor. mild upgrades.I finally got it tuned by a shop and they are telling me I need to open the exhaust up and see if this is causing my performance issues. They checked everything on the motor to make sure it was correct. timing, tdc, timing chain, valves,compression, coil bind, etc... cost me $400 for them to tell me everthing is fine and they cannot figure out the loss of power other than it could be that im S.O.L because I cant put a set of headers on here because of the early A-body unless I re-cut my fenders out again and put headers on or its my msd box. Im going to send my msd in for inspection but im curious about opening up the exhaust. when i got the car running I had the pipes on, no mufflers because I didnt have my welder. It performed really well. After I put the mufflers and new 2 1/2 pipe on my performance decreased. I am running 2 1/2 piping from the factory manifolds to 2 flowmaster delta 40's which exit right b4 the rear axle w/turn downs. I was wondering what is to be gained or could be damaged by having the exhaust shop cut my exhaust off and have 2 1/2 or 3 inch pipe from the manifolds and exit by the front or rear tires with no mufflers. i know its going to be a loud S.O.B but im running out of options. any opinions, criticisms, alternatives would be greatly appreciated. thanks for taking the time to read my long post.