The only nose mounted guns in the P-400/P-39 were 20mm Hispano and the 37mm Oldsmobile gun.
No version of the P-39N/Q ever had anything other than a 37mm in the nose. The only varients to have the 20mm option, with its very limited 60 rounds of ammo, were the P-400, and P-39D-1, and a small # of the P-39Ds (several versions) that were field modified in 1942 with non standard 20mm's, mostly in New Guinea and Gualdacanal.
The P-39Q is the best over all varient of the series, for performance, and power vs weight.
Embrace the 37mm. It loves you.