Eric,
No serious aircraft engine uses a spur gear reduction anymore.
First: aluminum gear case expand with heat increasing the gear back lash.
Two: line contact length is limited so there are high stresses in the gear.
Three: gear separation forces are high causing unwanted deflection.

A planetary has none of these problem. Line contact length is three to six
times gear width depending on the number of pinions. Some large radials use 18.

That is why planetary reductions are much lighter
than spur gear boxes. You will also need a torque isolator between engine
and box. Particularly the one rotor as torque pulses are severe.