You can't fit a multi-barrel gun in between the cylinder banks of a Merlin or Allison V-1710 without changing a serious amount of ducting on the engine. It is theoretically possible to do with the Allison, but you would have to reduce the size of the valve gears and relocate the induction housing somewhere else. The end result would be a significant increase in frontal area and a completely altered engine footprint, for little benefit. A 20mm Hispano type cannon is going to be more effective than a .303 Gatling style gun anyway.
For me the best armament fit for a radial engined fighter was the Fw-190A6 onwards, with 4 MG 151/20s (20 mm) in the wings and 2 MG 131s (13.2mm) in the cowl. This was enough to take down any target in WW2 aviation. The only fighter that I feel equaled this was the Tempest V Serise II, with 4 Mk V Hispano's (short barrel, increased RoF) and 175-200 rpg. La-7 with 3 nose mounted B-20s comes close as well.
For inlines give me the MC-205 with 3 MG 151/20 cannons (one in the nose, two in the wings) and 2 12.7 mm Brenda SCOTTI HMGs in the upper cowl. Close second is the Spitfire IXe with 2 Hispanos and 2 M2 Brownings.