liquid thick

I have a 97 Cobra Convertible that was a gem until my brother in law drove it. There was a lower end knock the next morning and my wife wont let me punch him!!

I got a 98 Cobra engine and to replace it and when I got it back recently there is a serious exhaust backfire. The mechanic doesn't know how to fix it. In addition to the motor I upgraded to a
bbk throttle body
bbk cold air intake
msd coil and wires
ngk iridium plugs
pacesetter long tube headers
pacesetter x pipe
sct istx tuner

that's where my fall bonus ended.

Since the motor swap no check engine light is on, I have filled the tank with premium 91 and replaced the fuel filter. We have sprayed the engine while running with starting fluid to look for cracked lines etc and everything is sealed or the motor didn't rev.

The mechanic really "wants" to figure it out but isn't getting any results... and wants more money!!

Please advise