2.8 litre chev v6 runs better with 1and 5 wires switched but still backfires through the carb?

Answer 1

go through the whole firing order and double check the order.
123456 number one being left front cylinder or passenger side front 1-3-5 driversside front is 2-4-6 distributor number one is right in the middle to the front of cap and it turns clockwise so like six oclock on a clock is number one and then around 2 3 4 5 6 hope this helps you

Answer 2

Make sure firing order and timing are correct first. If the motor still backfires, you've got a problem. I would be looking at something to do with a valve or camshaft wear causing the backfire.