it would be about the same as removing a carbed engine and replacing it. just remove the whole motor and accessories and all the wiring that plugs into it. drop in the magnum. get a wiring diagram (factory!) for your 90 w250 and find the few wires you need to tap into, and solder/wrap the joints. you already have the efi lines and pump, just check the ratings on your 90 pump to be sure it puts out 45+psi. if so, your whole fuel system can remain, making it that much easier!
any other questions, feel free to ask!
doug
