I dont know what has happened during my tune up.The motor will idle & rev in neutal all day, but when i put it in gear & go to accelerate it starts backfiring & will cut off sometimes.

here's what I did this morning as a simple tune up:
changed spark plugs to stock ACDELCO, same plugs that were in it.
changed distributor cap & rotor, same cap & rotor style that were in it.
changed spark plug wires, did one wire at a time to make sure I didn't mess up firing order.
changed ignition coil to MSD ingnition coil
changed air filter

I've checked the firing order twice, it right.

I dunno what the heck I did, any help is very much appreciated.