I recently put an aft pulley on my 64 slant. It is used for the AC belt, I think in all slant years, even dealer-installed someone here said, though perhaps not after-market (Western Auto, Sears, ...) since I have a 74 slant w/ a dbl-V bolt-on front pulley. I posted photos in a slant AC thread. The aft pulley does sit very close to the timing cover, but doesn't seem to drag since I hear nothing when I manually turn-over the engine (haven't run it since). Insure you have the "oil slinger" inside the timing cover since the damper hub bottoms out on that. If you need more room, one could probably add a 2nd oil slinger (or washer) inside the cover.
