Never worked a 737, Have most experience on 747's,A few 757,767,727,DC8-9-10, Which APU would be the best fit?

 

737..anything bigger will hang out the back! But if you wait untill the 737 goes the route of the 787, the centrifical compressors will disappear and this will shorten it considerably so as maybe to keep the rear seat!!.Oh,,787 eliminated almost all of the bleed air systems, went electric....its a nightmare!