Marcus, thank you. Your plan is right on. There is enough time between the two events that you can treat them both as A races. Use the 70.3 plan for the May event, then take a week (even two) of recovery, then load the IM plan for the weeks remaining. Your fitness from the 70.3 is more than enough to allow you to enter the IM plan halfway through.
Yes, there is a risk of overtraining, that’s why you need the 1-week break in there, and two weeks is not too much. “Rest” does not mean complete rest. 2-4 hours a week with very little running, mostly swim and bike, is good.