Hi firejoel
I'm not sure if my post is of much help , but I guess it probably would be easier to just fill the opening for the single turret with putty as you already suggested and drill/cut/file two new openings for both minigun and grenade launcher . I don't know of any resin sets available in 1/72 scale , I do have cobra company's 1/48 AH-1G back date set which includes a new resin turret .
On another issue ; I have two 1/72 matchbox AH-1J's sitting under the bench , I put them together a while back but the project stalled when I noticed that the oil radiator beneath the two exhaust cones is undersize and doesn't reach the outsides of the engine housing .
I'm not sure how to fix it now , I may have to try and carefully remove both radiator pieces and add some plastic strip to each side and sand and contour eveything in and hopfully put it back in position .
If you don't already have it , Squadrons AH-1 Cobra walk around book is a great resource .
Sorry I could not be of more help , good luck with your build , perhaps some day I'll get around to finishing my two Cobra's .
John .