With the way the XJ is designed, specifically the unibody, I don't think it would be easy. The 4BT will produce ALOT of torque. You'll have to plate the entire "frame" in the front. You might end up twisting the motor right off the unibody.
Don't get me wrong, I'd LOVE to see that swap. But I think starting with a vehicle with a real frame would be a better idea.
I did read an article on a LS1 powered XJ. Probably had to do the same supports you're thinking of. I will dig around to see if I can find it.
Just my .02
Bobby
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