The wingtips weren't chopped so much as redesigned entirely. They didn't just (literally) stick plugs on the wing of a 47D. The entire thing was redesigned to act as a wet wing. It was built new. For example, the ailerons are actually larger than those found on the D/M. They reach out further.
As for the gun emplacement, from what I've been able to tell, trying to overlay wing profiles from the D40 and the N, the innermost #1 and #2 guns on the N start about where the #3 and #4 are on the D so that they're superimposed over each other.
A little info on the developement of the "N", might help with this discussion!
The P-47N version of the Thunderbolt was the last version to be manufactured in quantity. It was a specialized long-range version built specifically for service in the Pacific theatre.
Four P-47D-27-RE airframes (serials 42-27385/27388) had been taken off the production line at Farmingdale and fitted with the Pratt & Whitney R-2800-57(C) engine driving a larger CH-5 turbosupercharger. This engine could produce a war emergency power of 2800 hp at 32,500 feet with water injection. These aircraft had been redesignated YP-47M and served as the prototypes for the P-47M series.
However, the war in the Pacific required fighter ranges even greater than did operations over Germany. In pursuit of better long-range performance, in mid-1944 the third YP-47M prototype (42-27387) was fitted with a new "wet" wing of slightly larger span and area. The aircraft was redesignated XP-47N. For the first time in the Thunderbolt series fuel was carried in the wings, a 93 US gallon tank being fitted in each wing. When maximum external tankage was carried, this brought the total fuel load of the XP-47N up to an impressive 1266 US gallons. This fuel load make it possible for a range of 2350 miles to be achieved.
The new wing also incorporated larger ailerons and squared-off wingtips. These innovations enhanced the roll-rate of the Thunderbolt and improved the maneuverability. The dorsal fin behind the bubble canopy was somewhat larger than that on the P-47D. However, the increased fuel load increased the gross weight of the aircraft. In order to cope with the increased gross weight, the undercarriage of the XP-47N had to be strengthened, which increased the weight still further. The maximum weight rose to over 20,000 pounds.
The XP-47N flew for the first time on July 22, 1944. Such was the USAAF confidence in the Thunderbolt design that they went ahead and ordered 1900 P-47Ns in June 20, 1944, even before the first XP-47N had flown.
The P-47N was destined to be the last version of the Thunderbolt to be manufactured. The first P-47N-1-RE appeared in September of 1944, and 24 were delivered by year's end. The P-47N-5-RE and subsequent batches had zero-length rocket launchers added. The R-2800-77 engine was installed in late production models such as the P-47N-25-RE.
The P-47N gave excellent service in the Pacific in the last year of the War, particularly in escorting B-29 Superfortress bombers in raids on the Japanese mainland. P-47Ns were able to escort the bombers all the way from Saipan to Japan and on many other long, overwater flights.
A total of 1667 P-47Ns was produced by the Farmingdale plant between December 1944 and December 1945, when the Thunderbolt line finally closed down. 149 more P-47Ns were built by the Evansville factory. V-J Day cancellation of 5934 Thunderbolts brought production of the type abruptly to an end.
Performance of the P-47N-5-RE included a maximum speed of 397 mph at 10,000 feet, 448 mph at at 25,000 feet, and 460 mph at 30,000 feet. Initial climb rate was 2770 feet per minute at 5000 feet and 2550 feet per minute at 20,000 feet. Range (clean) was 800 miles at 10,000 feet. Armanent included six or eight 0.50-inch machine guns with 500 rpg and two 1000-lb or three 500-lb bombs or ten 5-inch rockets. Weights were 11,000 pounds empty, 16,300 pounds normal loaded, and 20,700 pounds maximum. Dimension were wingspan 42 feet 7 inches, length 36 feet 4 inches, height 14 feet 7 inches, and wing area 322 square feet
