Sorry this is somewhat long.It may be acting up because of it being a SD setup with an aftermarket cam?? A cam is usually the main bolt on that can cause driveability issues in a SD application.The other bolt ons you listed normally dont cause an issue with SD.As far as timing goes,youll never get it right unless youve got a timing light.You cant really do it by ear or feel on these cars,if you want it to run correctly.I would double check your distributor indexing visually and your timing, with a light,to make sure you dropped the distributor in correctly.
1) Pull the #1
2) Draw a mark across the balancer at the 0° & 10° btdc position (using Witeout)
3) Use a torque wrench or breaker bar and 15/16" socket to rotate the crank CW til compression is felt at the plug hole (use some tissue paper stuffed into the plug hole slightly if you don't have somebody there to hold their thumb over the hole while you're rotating the crank)
4) Once you feel compression or the tissue blows out,rotate a tiny bit more until you get the 0° balancer mark aligned with the timing pointer then stop
5) Remove the distributor cap.The rotor should be pointing at the #1
post on the distributor cap.It should be pointed just like the picture at the bottom shows,slightly to the left of the pip sensor magnet,down inside the distributor
6) If not,the distributor isnt indexed correctly,which just means its not dropped in correctly (i.e.- 1-2 teeth off)
7) Pull the distributor and restab it until its correct.Once correct,reinstall the plug/wire, connect a timing light,start the engine and let it run long enough to warm up.If its idling too low to run & it stalls,youll have to loosen the distributor bolt just enough so you can advance the timing a few degrees (which will increase the idle) then restart it,pull the spout plug,aim the light at the balancer and advance the distributor til the 10° btdc mark is aligned with the timing pointer,tighten the bolt,recheck it with the light o make sure it didn't move when you tightened the bolt then reinsert the spout plug and you're done
8) The ecu advances the timing with the spout plug in place,so you have to make sure you remove it when you're setting base timing,otherwise you'll have the timing way off
So yeah your backfiring issues could be related to timing,but they could also be related to the cam + SD combo too.So I would say check timing really good,with a light,and reset if its incorrect and if that doesn't do the trick,you may have to do the maf conversion before the problems will disappear.Youll likely get mixed reviews on the cam vs conversion subject where some will say the cam will work and others will say it wont,but it really is the truth either way and dependant on the car.Some work and some dont with a SD ecu and a cam.
I assume when you said its a regular 5.0,you meant its a 5.0HO SD engine and not a 5.0 Non HO engine,right?? 13726548 firing order??
Since you added a few bolt ons,a 150lph fuel pump would be a good upgrade after you do the maf conversion,which I truly feel is the source of your problems.The 19lb injectors youve got are good up to 304hp (@ 100% duty cycle) but you really dont wanna run them above about a 85% duty cycle to be safe,which would mean theyre good up to 260hp and a 24lb injector is good up to 326hp at 85% duty cycle.If you have plans to do any more upgrades,24lb injectors and a maf sensor calibrated for 24lbs would be something to keep in mind before/when you make the decision on which maf sensor to buy,for the conversion.BTW - the 89-93 Foxbody oem maf sensor is a 55mm unit. Instead of it,you can get a oem 70mm maf sensor from a 94-95 5.0 or 3.8 Mustang, which are far more likely to be available at your local Pull-A-Part,if you wanna save some cash & not buy a new/remanufactured unit.A reman unit is around $75 I believe?? If you use either sensor listed above & you don't wanna have to get a chip burned to use the sensor,you'll have to use 19lb injectors with it.If you go 24lb+ with an oem maf,you'll have to get a chip burned.If you want a bigger maf sensor than 70mm (73/75/76/ 80mm) you'll have to get one thats already calibrated to whatever injector size you're running or get a chip burned to make the maf to inj combo work together.The link at the bottom will tell you everything you need to know about maf sensors and numerous other components.
As far as the egr & smog system goes, deleting those components wont cause any driveability issues or a cel (check engine light) on your year model.When the egr valve is opened,the ecu leans the fuel mixture and advances timing.If at anytime,a fault or code pops up for any egr system component,it will trigger the ecu to delete egr system function in the ecu's logic & the egr system components wont work and the fuel and timing values will remain normal.However,if the egr system components are still in place with the vacuum hoses still connected & the egr valve is stuck open due to carbon buildup or a defective evr solenoid,exhaust gases will still enter the system,but since the egr function was deleted in the ecu logic,the ecu's lack of leaning fuel and advancing timing will likely cause driveability issues.If the egr components were removed completely,you'll have no driveability issues related to their deletion.The same goes for the smog components.Just make sure all vacuum hoses are removed/plugged for both systems.It also wouldn't be a bad idea to pull codes.All you need is a jumper wire and a 12v test light.The second link below will show you how to run the tests.Use the test light connection method in the link diagram since you don't have a functional cel.The codes will be flashed through the test light.If you run a cylinder balance test after the code tests,it may help you locate a suspect injector......
The Hot Rod Garage - Mustang EFI Information