Well, the hard part is mounting the compressor and routing the tubes. But they have that covered. 4.0L alternator mount is the same as the 2.5L. Would mean that you could mount the compressor in the 2.5L but you'd still have to figure out air/fuel management.
I notice that engine is a Renix. It's doesn't use the standard AC/alt mount. I think it would be pretty straight forward to mount one to a >'91 setup. Would be easy to remove too. If I had more cash and access to a welder, I'd try it.