Need help with Accurate Miniatures SBD-3 modification

The AM SBD-3A kit (480311) has the twin gun parts included.  Also the Aires cockpit (4212) contains the twin guns. 

Aside from the gun mount, ammo stowage needs to be changed also. The single .30 was fed from an ammo can attached to the pintle mount of the gun. The twin mount .30s were fed via belts from a larger ammo stowage can at the rear of the gunners compartment.

twin mount

single mount frame. the preforated area is the mount fo the ammo can

single gun stowed

and a top view without the single mount installed. On the twin mount SBD, the ammo stowage goes in the bottom area of the photo

...what would be the easiest and/or cheapest way to put the correct configuration of the twin 0.30's in the rear seat...?

Hiya, Fred... My opinion on the matter at hand, based on experience, is this: The cheapest way is too scratch-build both guns, using the kit part as a pattern... Or, you could fabricate the just guns' recievers and ammo cans and display them stowed, negating the need for barrels and ring and bead sights... (The ring-sights are folded down against the upper receiver, or removed and put in the gun-bag when the guns're stowed)

The easiest way is to cast a resin copy, or like what was mentioned above, buy another kit, or after-market parts-sets...   I'm the guy that always goes with the cheapest way, lol, since you can't have both cheap and easy ...  But there are varying degrees of "cheap"...

Speaking only for myself, if I didn't cast a second gun and ammo can/feed-system from resin, I'd scratch-build the guns with styrene strip and rod (or stretched plastic Q-tip handles with wire inserts (if you want hollow gun-barrels)... The .30-cal Brownings are pretty easy to fabricate since the basic shapes of the barrels and receivers are available... well,  everywhere...  I'd build them as one piece though, rather than try to build two identical guns...

Trust me, it's easier to glue stuff that small together and then trim it to fit, than it is to measure it, fit it, measure it again, then cut it, and then finally glue it...  Then notice that you got a measurement wrong, tear off the offending part, then make a new one...

Casting parts, for me, is the cheapest easy way to go, but you gotta shell out a few bucks for the materials.. A "Starter set" casting kit, containing everything you need to get going, runs about 30-35.00, so you want to cast more than one gun for one kit one time if you go that route... That's the only way to make it cheap(er) than buying after-market parts or second kits...  Myself, I cast all kinds of detail parts (like guns and ammo cans) and figures that can be used on other kits, so my unit-cost is pennies per part, eventually...