The .50s are for the waist gun postions, the .30s for the Eyeball turret in the nose. Another .30 was carried in the tunnel gun position (aft of the waistgun position bulkhead), but you won't see much of that through the blister windows (trademark feature of the PB2B-1) so a PE'd kit gun will do there.
Studying references, most kit windows will need reshaping and/or repositioning, aswell as adding 3 others, including the two blister windows in the tunnel gun position associated with the PB2B-1.
Not making any progress here at moment sorry, the little time I get for modelmaking is spent f'ing round trying to get the Bf 109H-1 cockpit to a standard I'm happy with.
If I'm unable to do anything soon, I may move this to the 'start and finish' threads instead, as continuous unfinished GB entries is not good for my head.