the 2.8 will move that MG just fine but it will still seize up due to oiling problems. Wish I could help, the first 2.8 I'd seen in years showed up at the pick n pull like a month or two ago but it's long gone now
I know the 3.4 is a common swap into 2.8 equipped XJs, is there any chance a 3.4 intake manifold will fit it? Don't know the old stuff as well as I should, but I'm certain that 3.4s are much more common than 2.8s in the junkyard, so if the manifold off one will fit you should be golden.