Another urban myth.
The XJ unibody is plenty strong enough to handle a small V-8. Remember, the XJ was designed by AMC, and AMC had been building unibody cars since the mid-50s. The super 2-seat AMX, with its 390 c.i.d. V-8 that would turn mid-13 quarter miles in stock trim, was essentially the same unibody as a Rambler American or a (Gasp!) Gremlin. The XJ unibody isn't that different from a ZJ, and they stuffed a 5.9L V-8 into that.
The V-8 conversion has been done. The most popular seems to be the small block Chevy, although I see a lot of people "thinking" about a 5L Mustang engine (haven't heard of anyone actually doing that one, but it must have been done somewhere by now).
Check Advance Adapters' web site.