Hi Marc,
You have to make sure that the foam that you have used is not flammable, and will not be affected by the high temperature of the engine bay and the heater box. Perhaps it would be better if you also could screw the new fan assembly to the heater box.
Something else to consider, is to seal up the rear opening of the fan cage. The P6 heater box is designed to suck air from the front chamber only, that is fed from the air intake between the bonnet and the windscreen. If you leave the rear open, There is the posibility that the fan would suck also air from the engine bay, and this is something to avoid. Alternatively, you can take advantage of the rear opening of the fan cage, and use it to create a recycling facility when the main air intake flap is closed. However, this is going to be more complicated, and to be honest and don't find recycling very useful, even in moderns.
Regarding the electrical connections, you need to pull one more wire from the heater box feeding harness, to the blower switch in the centre console. I think that you have to remove the console completely in order to reach the internals of the switch. Another option would be to fit a new 4 position switch, or just use 2 of the 3 available speeds of the new blower motor, for example 2 and 3, if you check and decide that this fits your needs.