where gear oil goes in the back and it sounds like it is misfiring

The fill plug for the differential is at the front of the differential, above the centerline, on the left side. Check the distributor cap for contamination or carbon tracks both inside and out. The distributor should be covered by a rubber diaper as supplied by Ford. Also, check the firing order of the plug wires. Mixed up plug wires can cause misfires and popping. The Mustang used a different firing order than other Ford passenger cars.