[SubyTechJoe]

In terms of exact fit, the engine you need would be an EJ25 from a 2000 thru 2004 Legacy, Outback, 1999 thru 2004 Forester and some 1999 thru 2004, 25RS Impreza's will fit. I would be careful MT and AT as some of the blocks are not drilled for both. Best plan would be At to AT or MT to MT for ease of install

 

[SubyTechJoe]

Be aware some of the early AT blocks wont work because the Engine block is not drilled for the lower starter bolt as the lower starter on the AT is a stud on the Transmsson case (I have personally encountered this on an engine installed by another shop)

As to the issue of the belt guide situation, the guide exists to keep the belt from jumping time in case of a push start, which would not happen on an AT motor. In terms of need, it is not needed on an AT car but causes no harm to have inless your worried about the extra ounce of weight.