1977 cadillac backfires

i have recently got a 1977 cadillac coupe deville with a 486 supposibly and a 4bbl carb... now my problem is that she will not stop backfiring...now when you are sitting in a driveway in park she wont backfire if you rev her but the minute you put her in drive and pull away she start to back fire you can accelerate without backfiring... i was thinking it might be the timing but not sure what the fireing order is and what the number one post is on the cap if you can help it it would be swell. thank you

JARED

First thing you need to do is research the info about your car ,it has a 429 engine ,downsized like the car from the 76 500 incher. When did this start, any work done before it started? Get a shop manual or do a google search for manuals, E bay is good also, you will need one to keep up with this car.