Stock timing set-backfiring

[USMC302]

1st, just because it's back firing, doesn't mean your timing chain is messed up . The timing chain is standard for both stock and aftermarket cams with exception to different placements of the lower cam sprocket for advancing or retarding the camshaft itself. Unless the chain is extremely stretched out, the timing issue is NOT from the chain. Did you install it straight up? You might have installed th dist 1 tooth off or at 180 degrees off, the car will run like that but not too well. Pull the dist out, set the #1 piston to TDC, you can see this with the spark plug out and a flashlight. Once it's at TDC, drop the dist in with the rotor pointing to #1. Set your timing to 10 degrees and go from there. This is the warning you are referring to most likely.

CAUTION: Use Edelbrock Performer-Link Timing Chain and Gear
Set #7811 or Accu-Drive gear drive #7892. Do not use late
model timing chain and gear sets that are designed for emissioncontrolled
engines. These timing sets are machined in a retarded
position and are not recommended for this camshaft installation.
Edelbrock Timing Sets feature three keyways for specified timing
selection. Use "0" position for most applications.

That "0" position they are referring to is the straightup position, which is going to be almost the exact same as straightup on the stock chain. Try resetting the TDC and let us know.