Any time I have a misfire on a car with a Crane ignition, especially a misfire that gets worse as the car warms up, the first thing I do is verify that the optical trigger is phased properly. What I mean is that I make sure that the trigger is signaling the coil to fire when the rotor is centered on the contact point on the distributor cap.
Because these Crane ignitions are generic units with the trigger on a sliding bracket, this is not a given. I've experienced this problem with a number of cars that have Crane ignitions. Sometimes, a new set of plug wires will make it more noticable because some newer sets of plug wires currently in the supply chain have higher resistance than older and/or original sets. Sometimes, one cylinder will miss worse than some of the others because its plug wire is closer to a groud source.
When a Crane unit is out of phase, the car runs OK when cold because the spark is able to jump the gap between the rotor & the contact point on the distributor. However, as heat increases, so does resistance. Therefore, as the car warms up, you develop a misfire.
The best way to check for this is to drill a hole in your old distributor cap (if you still have it) just inside of & directly in-line with one of the plug wire posts. Install this distributor cap on the car, & hook up a timing light to the plug wire that corresponds to where you drilled the hole. Then, with the car running, put the timing light to that hole. If the optical trigger is phased correctly, you will see the rotor pointed right at that post on the distributor cap. If it's even a little off center, the the optical trigger is not phased properly, & you have found your problem.
I belelive Crane's web site has all of this information, as well as directions for phasing in the trigger correctly should you find it necessary to do so. It's one of those tasks that is quite simple to perform yet for some reason requires an overly lengthy explanation.
If you do have to adjust the phasing of the trigger, this will most likely affect ignition timing; so you will have to check & possibly adjust ignition timing after you have the trigger phased properly.