While you can’t use an XT’s alternator in an XT6, you can give an XT an XT6’s alternator, but it might not be worth the effort. It can be done, but not without serious modifications in the use of an XT6 alternator on an XT. In the case of using an XT alternator in an XT6 you simply can’t. The reasons for both answers lie in the fact that the alternators are actually quite different. The XT’s 65amp alternator is designed with a “double” pulley (to drive two belts). The XT6’s 90amp alternator has a much wider pulley to drive a single belt; it has an internal fan, something the XT’s 65amp unit lacks. The modifications required to use the XT6’s alternator in an XT (or another EA82 powered Subaru) are probably not necessarily worth the trouble, but it can be done. However, unless you just want to be able to say you managed to perform such a change, it’s just not worth trying. Get the right alternator. In the case of the XT you can scavange virtually any member of the early to late 80s Loyale/Leone family for it. In the case of the ER27 powered XT6 or Alcyone VX you’ll need to either find a junked twin or go to the parts store. Remanufactured alternators can be found for about $130US, some with lifetime warranties. A new alternator from a Subaru dealer runs about $500US or more.
A word of caution
Replacing the alternator on an XT6 is physically one of the simplest repairs there is, but always make sure you disconnect the battery first! Do not reconnect the battery until the alternator is fully installed - otherwise you run a very high risk of a very painful electrical shock and/or ruining the alternator. When cleaning or checking the connections disconnect the battery as well - one slip and either of the two little wires on top can come into contact with each other or might touch the alternator housing itself. If this happens you can be assured of a couple of very big sparks, a crackling sound, a bit of smoke, and almost certainly a ruined alternator. Another surefire way to ruin an alternator (and possibly more) is to incorrectly connect (cross connect) jumper cables… that’s how ten years after buying my XT6, after 25 years of jumpstarting car batteries, I finally managed to kill my original alternator.