Ok , here we go I had this same problem for years working on first gen SBC. I came across this somewhere and it worked. the more radical the cam the less margin you have to work with. I have done these SBC for 41 years. here it is. follow it to the letter and it will work. I did not write this only copied it.
Setting Valve lash
Centered on the base circle EO/IC
Exhaust opening/intake closing
When the exhaust valve just starts to open, stop cranking and adjust the lash on that cylinders intake valve.Crank the engine again watching the intake valve of the same cylinder. After it is fully open and the rocker tip is at the bottom of its travel crank slowly until the valve is 2/3 closed. Stop and adjust the exhaust valve on that cylinder. repeat on each cylinder.
With hydralic lifters I give them 1/3 to 1/2 turn from "0". not the factory spec of 1 full turn for lash.
This may be time consuming but it works. I remove the spark plugs when doing it to stop crank back from compression. I also use a remote cranking switch, ignition off .I then set the distributor to #1 at TDC checking the timing marks on the balancer. Then fire it up. Set the timing with the light, vac advance plugged. Check the weights in the disrributor for binding first. Sorry forgot that one. time it 6 to 8 degrees. Warm it up and run it at slow speed to purge any air from the lifters.
Now after all that put all the tools away but the wrench for the distributor. drive the car. hammer it, slow take off , do it all. and listen and use the seat of your pants to fine tune the timing. when you get it just right you will know. Just do very small moves on the distributor.
I think you have an exhaust valve hanging open. It is easy to do.
I use to do this for the guys with the door shut. You know one of those little secrets to piss them off
By the way any fire is a back fire. I never heard that other one. I go with miss fire, but the other sounds like politically correct bull.
That should fire some one up