Well, I have hopefully managed to sort this for now, although time will tell.
I've JB Welded it together again and left it more than 24 hours to cure. It feels strong, but we'll see how it does after a good heat soak.
One other thing I forgot to mention -
Before sorting this, the same times when my Wagoneer would struggle to start, it would always run rough until it had been driven for a few minutes. It would hesitate too, meaning pulling into traffic wasn't fun.
I tended to avoid using the Wagoneer if popping from store to store, as it would basically run rough the entire time.
Now the carb is vented again, this has totally gone - it runs and starts like a new car in all situations!
1991 Grand Wagoneer - Hunter Green. All stock. Rebuilt 360 with Melling MTA-1 cam.
1998 Cherokee (XJ) 4.0
1997 Grand Cherokee (ZJ) 4.0
1974 Ford F100 390