1968 Ford Mustang car set for long time now trying to start

First thing to do is take a compression test wet and dry. That will tell you alot. Check your spark plugs/wires and distributor/rotor cap. They cap may be cracked and wires or plugs may be bad. You didn't say if you replaced those or not. You should have though. Then check your vacuum at idle and see what it is. AFter doing all of that you pretty much should have a handle on it. Unless it' s a more serious problem. If any of your cylinders vary by 20% then you may have an internal problem.