When the backfire happens, is it just when you initially rev the engine, or does it continue to backfire when the rpm's are up there around 3000 or more? If it's just on initial rev, I would think timing's a little to far advanced or the power valve in the carb is gone. As a matter of fact, I'd almost put money on the power valve unless you have a power valve blowout preventer. The power valve is designed to dump fuel into the carb on initial rev up. But in earlier design holley carbs, if you backfired through the carb, the power valve was toast.
Now then, on the cam, at .50 your intake is 275 degrees and exhaust is 288 degrees duration.(my best guess with int. opening BTDC and closing ABDC, and the exh. opening BBDC and closing ATDC) This is definately not a street cam profile. You'll be lucky to idle around 1200rpm. If you have power brakes, you'll need a secondary vacuum source. One thing you didn't mention on your cam profile was the LSA (lobe seperation angle). I would assume that it would be around 102 degrees or so. I would bet the HP would come close to 500 especially if the heads have had any work done to them.
How do you plan on driving this car? This is definately a strip engine as your HP band is 4500 - 7500. My suggestion would be to check your overall timing (distributor+crank) and if that is good (11-12 at the crank and about 24-26 at the distributor) then I'd look at the power valve.
67 Camaro LS6 454/TH400/12bolt 3.73