UPDATE: Truck is just about fixed so the jag can be rescued soon from the wrecking yard. I've decided that the best option will be to get a cheap donor vehicle. My plan was to keep the 4.2 and mate it up to a GM 700R4 transmission. But as I look around, I'm tempted to just buy one with a V8 swap already as a donor. Part of me really wants to keep the 4.2, but I LOVE the power from the SBC.

 

Personally, I would stick with a jaguar motor. There must be businesses that can provide updated fuel injection and electronic ignition for the 4.2. If the motor has a heavy flywheel, (a legacy of Leyland diesel engines), throw it away and use a proper, light flexplate for the auto. With the correct gearing, and that 4 speed auto, I think you will find that the old girl gets up and goes quite well.
good luck
Pete