Backfiring question?

[Red90GTDude]

Ok I am not a super techie so I will do my best. I have a 1990 5.0 Gt 5 speed, Pretty much stock. I am the Second owner with rather babied 145,000. I've had her since 40,000 and do L.O.F ever 5k-7k.
After doing some reading on the net, I took the airbox silencer off around 85,000. I Recently put on a mac 2-1/2" offroad H-pipe, 2-1/2" intermediates, some v-force mufflers, and 2-1/2" tails. Then I put on a 70mm Performance Products T.B. and opened up my stock intake manifold opening to about 69mm(was pretty slow getting it there with a dremmel and $4 cutter tips LOL) I also have my timing advanced to 14 degrees. Since these changes I get a small backfire sound near under where the hpipe is. It is mostly a cold problem but sometimes hard acceleration too and doesn't happen that often. When I put on my new C&L 76mm mass air, it gets quite worse with a LOUD BANG backfire in the same general area... sound wise that is. My exhaust guy says a throttle bodied car shouldn't backfire unless there is a "leak" somewhere? I asked him where he thinks it cold be? and he just kinda shrugged and said who knows?
So that is why I am here? Does this exhuast leak sound right or am I getting to much fuel to air mixture? If that is the case how do I solve it?
Other info. I have rather new 10k bosch platnum2 plugs, new cap & rotor, new fuel filter, recently cleaned/oiled k&n air filter. Real Old (like 100k)8mm Twin Core SplitFire wires. Other than the bakfires I REALLY Like the Sound & Power gains! Thanks very much for the time.
Tom