Yeah, I am totally confused too.
I won't argue with you about the cross over since my original line of thinking that a threaded plug might be the issue is moot.
The crank case needs to either vent into the atmosphere or, via a PCV system, have the gasses scavanged into the intake to be burned. If you are doing neither, you are building up positive pressure (heat and blow by) and the weakest seal point will fail.
So you have a pcv valve in the intake. It has a hose connected to it that is routed to the base of the carb so that the air in the crankcase can be drawn into the combustion chambers and burned with the normal air/fuel mixture. RIght? In order to allow air flow into the crank case the oil filler cap has a connector that allows air to be drawn in. The connector usually has a hose on it that routes to the air cleaner.
If you don't have this setup, or it is not working correctly, then the crank case might be experiecing positive pressure. The pressure may be forcing itself out the ends of the intake which is probably the weakest sealing point.
82 Cherokee WT - SOA/SF/agr box/Borgeson ujoint steering shaft/401/performer/holley TA/HEI/BeCool/727/np208/d44(aussie)/amc20(ARB)/BFG 35X12.5/Corbeau Moab Seats/RCI 6point Harness,Hella Aux lights/tuffy console/killer32 slider bars/custom bumpers and roll bar/Alpine head unit/amps/speakers 2002 Liberty Limited Early edition - Basically stock