taking the windshield washer reservoir off for an intercooler doesn't seem like a good idea to me. the problem with the top mount intercooler is the pressure areas around the car. people put the intercooler in the front for a reason, it gets the most airflow going through because of simple physics. a top mount intercooler has less pressure because there is a lot of air flow over the area, so in order to make the intercooler effective, it would have to stick up a bit out of the hood area. the lesson is this: can it be done? yes of course, is there a reason people don't normally do it? also yes of course. also consider the Subaru only has a 4 cylinder engine, and doesn't need near as much cooling as a V6.
if you aren't going to run headers, don't bother with a 3.4 pulley. there is another physics lesson, what goes into an engine, must come out, or problems like a chipped piston are going to arise. you could install an intercooler like the one Scottydoggs has, and use it for a turbo, another member used a similar intercooler with other mods and ran a 2.6 pulley, and had plenty of cooling, then switched to a turbo, and it still cooled the air very well.
if you plan on running a turbo at some point, do yourself a favor and keep it simple. there is a reason the large majority of people have a simple front mount intercooler.