P-400s never had a 37 mm to donate. The 37 mm cannons came from P-39s. IIRC, these were P-39s that were no longer repairable.
Well if nothing else, the P-400's on Guadalcanal donated many of their 37mm cannons to PT Boats.
I personally like the look of the P-39, but freely admit it had several disadvantages. One not mentioned is that the car doors could not be opened very easily if the pilot had to bail out.