He verified the firing order with the vavle covers off, which is easy enough.

Aftermarket cams for a 302/351 will specify the firing order. Cams that are listed specifically for a 221-302 will be 302 firing order (1-5-4-2-6-3-7-8). Likewise, cams listed specifically for a 351 will be 351 firing order (1-3-7-2-6-5-4-8). Roller cams for the 302/351 are generally ALL 302HO/351 firing order (also 1-3-7-2-6-5-4-8) unless they are roller conversion cams (221-351), then it will specify either/or.

Example.... http://www.cranecams.com/?show=promo&id=37

 

If you running points then it sounds like a bad condenser. Also if you have a Holley carb the power valve is bad due to the back firing now. Also check the fuel in the bowl for correct height, might be set to high or even stuck causing raw fuel to get sucked in. Might even try a new distributor cap and rotor. A cracked or dirty one can cause a cross fire. Make sure all the vacum ports are plugged even check behind the carb near the base and check the intake too.

Also take a timing light and hook it up to each lead one at a time and see if the light pattern is the same. Easy way to see a misfire or no fire at all.