I don’t know about the '84, but the 85-7 XJ6 block does have engine mounts further back, and you will have to fab new mounts if that is the case. I have 2 '87 engines and considered this swap seriously for my Mk9, given all the benefits of the later engines including Bosch fuel injection. BTW, I have an XJ6 sump on my 4.2 Mk10 as well as XJ6 exhaust manifolds and the stock exhaust pipe clears the sump with no problem, so I don’t see that as an issue on the 9 unless it’s with the steering shaft, but I haven’t direct experience. You would also have to modify the inner fender wells, at least for the s3 engine. Unless the '84 has the older block, it gets to be too big a project, as has already been said. Someone on the XJ6 forum could answer that.
I ultimately decided that lumping is the best option for me, since I want an upgraded transmission, and as pointed out already, the narrow front of the Mk9 engine bay is a problem. Now, when I can do that is a big question, since I have three cars under restoration right now. So, please stay in touch if you decide to do either conversion.