I finally got the 429 in the 66 and running. Now, the motor will backfire up through the carb randomly. No rhyme or reason, just a big backfire and the engine dies. I have good fuel pressure and stock dist. with pertronix. Stock block and heads, XE 274 cam, perf. rpm intake, and a holley 850. I checked the timing twice and it is set to factory specs. Any ideas?