slant 6 troubles

where did you set the timing?
Backfire through the carb is usually caused by retarded timing or a lean condition. Is there a good shot of gas from the accel pump in the carb?
Whan I had the/6 in my car, the factory timing spec was like TDC or maybe 2 degrees ATDC, but I set it at 5 before and never had any problems. Just some thoughts, and don't give up on 'er!