Aires Quickboost has released a replacement for the Monogram (and later Revell) series of 1/48 B-25J kits that replaces the cheek gun packs. There are no instructions but this is direct a replacement for the kit part. No modifications to the kit are required; the assembly simply goes in the same place as the kit parts.
Quickboost has molded the external gun packs in light grey resin with no apparent bubbles on three resin sprues. Each Quickboost twin gun pack is supplied on its own resin sprue with a separate sprue for the four .50 caliber gun barrels.
Although most paints will adhere to resin alone, I would recommend that you wash this part to remove any remaining mold release and prime it first. This part will need to be installed with your favorite CA (super glue) or epoxy, as the normal plastic glues or solvents will not react with the resin.