I had forgotten that picture. Excellent example. The Dana has that auto-on thermostat too, a nice touch back in the day. Larry Pina shows the exact procedure and a similar picture as well for just the fan. If you don't have one already, Macintosh Repair and Upgrade Secrets is a MUST have for the Vintage Mac user and fairly easy to come by these days.
FYI, adding a fan requires adjusting the voltages to accommodate the added power requirements.
ALSO: The SE sold an optional floppy drive plastic shield back-in-the day as an "option" to reduce the amount of dust sucked through the floppy drive opening and eventually clogging the works. You would be well advised to create a similar enclosure around your Plus floppy drive to avoid future problems there, especially considering the venerability of these early drives to mechanical failure. Hard to believe it was an option based on the grotesque conditions I have found many of these drives from SEs which were never cleaned.