my engine idles in park but in gear sputters and back fires when you gas it! changed spark plugs and cap
miss firing on 1991 Ford Thunderbird
This sounds load related. There isn't any load on the engine when it's in neutral, but when you put it in gear, there is. This could be related to the ignition or fuel system, you'll need to have a diagnosis of both systems performed to know what this might be.
are you sure you have your spark plug wires on right