Same dilemma here. My IM roadster has a 2.4 911S engine installed but I am pretty sure I can get more money separating the two and installing a 911T engine i have sitting on a shelf or a VW. Almost time for that decision. Been over 17 years with this car.
What tranny do you have the 915 5 speed? or the 901 5 speed?
If you do then I would take the time to advertise it with the engine and tranny intact and only if I did not get good value offers would I consider changing venues and advertising the engine alone.
ON the other hand you could advertise the engine alone to see what you would get for it but for all the work of changing the motor it may or may not be worth it. My real estate agent once told me the value of your house is only established when you SEE the CHEQUE..