My carbureted jeep is the same way, I have a weber carb on it and I just put a high performance racing distributor on it last week (combines to make a beast out of the 4.2L inline 6). No matter what I did in the past some plugs simply came out fouled. There was no two ways about it and even though they were fouled they never really affected performance (no detonation, etc). One thing I do with it though is stay about from those small diode plugs, Bosch in particular, just to be sure.
My 308 (qv) I left the plus in for like 8K miles, pulled them and fouled somewhat, but not bad considering. 308s are supposedly prone to plug fouling, not sure how true that is, but have heard it repeatedly.