Gsoleski

I just got my 350 in and running. It seems to drive pretty well and has plenty of power. But the problem is when I shut it off it will diesel and backfire very loudly. It sounds like a gunshot. I thought this could be the timing, I had it at 15 BTDC and the car would idle at 1800 in park and 1100 in drive and would buck coming to a stop. I brought it down to 12 BTDC and let it run. 1200 in park and around 900 in drive but it would want to stall every 10 seconds or so. Even at 12 it would diesel and backfire, if not even louder than before. I don't want to blow a piston or any rings the engine is brand new! A little info about it, 355, vortec heads with just about every mod including larger valves and 59cc chambers, GM hot cam, RPM intake, flat tops with 4 valve reliefs, 10:1 compression with 93 octane gas, and a qjet on top. I know it has a .040 quench also and plugs gapped at .040. The engine has around 35 miles on it and honestly the backfire didn't start till after 20. Any help is appreciated!