Raggy, my bit of advice for what it is worth.

I have been giving illustrated talks like this for about a year now and my formula is;

The subject is immense so get a few songs together to cover the period/ subject you are going to cover. You have only forty minutes so you wont need many, with your extensive repotoire you should have no problem.Link the songs with speech, keep it simple and amusing .

Take your guitar,I am only a singer and manage better than well

leave room at the end for a bit of Q and A.

i saw Lee Kaufman at the Hull University doing a talk on Woody Guthrie coupla weeks ago and he followed this formula.....he had a fiddle banjo and a guitar as well as a superb singinging voice