It's even easier than that. You can use the B27 bottom end and B25 top end with no modifications needed. You will make about the same power as the stock 2.5L motor but with more low end torque. You will also need the wiring harness and DME from the B25 motor. Overall though it's not a whole lot faster than a stock B25.
There's a wealth of information here http://www.e30tech.com/forum/showthread.php?t=48288