Wow.just to answer as well - all of the engine harness for the 14cux can be left in place and simply tied up (you can even just unplug the ECU). The Holly Sniper EFI does not use any of it. And you Speedo is a cable running off of the VSS (used by the 14cux ecu but not the holly)..
The only thing that is going to have to change is the fuel pump, the sniper requires 60psi, the stock rover one does not make that much pressure. its fairly straight forward to pull the tank and replace the fuel pump though - something to think about.
So in theory when your ready you just pull the 250 out and drop in the 3.9 in a weekend if all the wiring and other 3.9 bits are still there.