Hello, I hope the following assists you.
The most common cause of a misfire on the Street KA is an ignition system fault, usually an ignition coil failure. You mention you've checked the leads, the leads should have a purple dot near the spark plug end, if not the other type of lead can give problems, this only applies to genuine Ford HT leads. Check the output of the ignition coil using suitable equipment such as nylon pliers, check the condition of the spark plugs. If there is no output for one cylinder from the coil then replace the coil assembly, if there is no output for two cylinders either one and four together or two and three together then check the coil trigger circuits.
Start with these points, let me know if I can assist further and I'll reply as soon as I'm able to.