I remember on the Full size Jeep site, their is a post about using an adaptor and a large distributor cap on the Motorcraft Distributor. http://www.ifsja.org/tech/electrical/ignitionupgrade.html
I believe you could also get the parts for the 6 cylinder distributor. You would have to get the parts for a 300 CID 6cyl in an F150. the adaptor and rotor were the same, but the cap was for a 6 instead of an 8.
the Same post also talked about changing the ignition coil to the motorcraft high output coil. It was kind of square instead of the normal "can of soup" style that was used at the time.
This was a cheap upgrade that everybody said was worth the money.
I believe that the distributor used at least on the Renix vehicles were made by Motorcraft, so the cap and rotor should work. But i wonder if anybody has tried to upgrade the coil on 4.0 Motors?
Could be good bang for the buck.