I can't really say if a RX4 gearbox would be a straight fit although I do doubt it from experience... as I said the sprinter transmission tunnel is very thin. Keep us (me) informed if it does bolt fit right into the tunnel.
The W58 is a interesting suggestion... I've never heard it before (13B AE86 with W58 that is) but it seems like it would be your easiest, cheapest and least difficult solution. Have you seen it done son57a