On a more sober note -- one of the guys near me who used to have an XJ did that swap, and afterwards he swore "never again."
The 4.3 is a 90-degree V-6 engine -- it is essentially a shortened 350 V-8, so it is just as wide as the 350 and requires pretty much the same adapters to make it work. Check the Advance Adapters web site for particulars.
The easier swap on an early XJ is the GM 3.1L or 3.4L V-6. These are built on the same 60-degree V-6 block as the 2.8L that came in '84 thru '86 XJs, so it's a better fit and a much easier swap.