The swap with a 4.0-4.6l physically is a simple replacement. The engine block is the same no major work required. The rest is wiring and or computer mods.
Look up the cost of any shop offering a engine swap and add about $1000.00 and that is a good ballpark. Your 4.0 is the same external engine block and the transaxle is the same so the only issue left is the interface between the cabin computer, engine computer and maybe some wiring harness issues.
Now comes the cool part.
The 1995 Aurora and caddy are very similar and we already can do the computer for both. So use the same sensors that came with the Aurora and same computer but modify it to work with the 4.6l. It "should" work just fine. Yes we would need the car to be 100% sure but if we have a connection with whoever was doing the swap then it would be fairly easy.
The L67 swap in a 4.0l Aurora is alot more work... and silly consider you would be stepping down for that car.
When all is said and done you could just rebuild the 4.0l and get the computer mod to match the build. You can easily get 400hp out of a 4.0l. I suggest looking up the Shelby Series 1 cars. Look at what he has said about the 4.0l.
If you go to the bottom of the linked page and click the play button you can get a great sound clip.