Considering fitting some 15x7 mags on my P1800E, car will be used for occasional track day.

Can anyone tell me if they will fit under guards

No experience myself.

This info from IPD might be helpful:

https://www.ipdusa.com/Articles/520/Wheel-and-Tire-Fitment-Guide-for-1962-to-1995-Rear-Wheel-Drive-Volvo-Models

I have a set of Multi-X wheels (15x7, 25mm offset) and a 72 1800E - can try to test fit them to the car next week when/if my back stops aching and present domestic emergencies are resolved.

Edit: forgot the YM of my own car....