Diff FS thread is here http://www.perth-wrx.com/vb/sale/40992-your-chance-bargain-lifetime.html

From his description it is a 3.9 from a STi RA, fitted by ASG to his 04 WRX.

Would this be bolt on for MY99/00?

It would be so much easier if I could find a chart of all this information, there is just so much variety and different combinations! :/