The 165 gal drop tanks does not help out with internal fuel situation for the P-51, whether it has fuselage tank or not. The remaining internal fuel takes precedence, it does not help much if any that we have 330 gals of external fuel (plenty of what is lost once DTs are dropped), while after combat there is 150-175-200 gals.
Tomo - I am well aware of the fact that operational combat radius is determined by internal fuel remaining, which is why I caveated the comment. PTO doctrine for VLR missions did not drain the 85 gallon tank which by that time, per Operation manual cited filling with 65 gallons and burning down to 25. The VLR missions carried the risk of engaging in combat with perhaps 65 gallons remaining after climbing to cruise altitude, then going to external tanks. The effect of retaining approximately 65 gallons instead of 25, and using the wing tank for warm up, take off and forming up was the same process with same amount burned off. The net is that with this process, operating under the assumptions for determining combat radius (combat, reserve, etc), leaves the P-51D with a 'bonus' 40 gallons at cruise (now at 210 IAS @65g/hr) or another 160-190 miles+ (radius miles) at the extreme.
Retaining fuse tank fuel was not uncommon, even in ETO with known CG issues. Entering combat was a tempered judgment with respect to maneuvering combat - versus engage and return in lieu of engage and pursue. The 355th FG, for a specific example, went to Posnan on May 13, 1944 with 75 gallon tanks, shot down 13 on the way and over the target and returned - 1470 miles...and retained most of the fuse tank reserve.
That said I have not seen the P-47N Manual to see if there is a comparison. As they had no CG issue my above argument is that the 47N burned minimal internal fuel in the climb to altitude, whereas the P-51D Mustang Manual deducted the fuselage fuel tank upside for the cg issue. BTW as near as I can tell, the P-51H Operating manual also burned down to 25gal when there was no Cg issue. The reality is that with a reduction of 60 gallons from the 85 in early pre-cruise flight segment, left the 51B/D with 204 (85+92+92-60) gallons whereas the P-51H could (and did) use nearly all external tank fuel save the 10+ gallons for warm up, take off and switch to externals. That left 250 (205+55-10) gallons and combat ready at target. Additionally, the P-51H had about 10% less parasite drag at cruise speeds.
You can note that AHT lists 700 miles of combat radius both for the the P-51H with 110 gal drop tanks and P-51D with 75 gal DTs. The 110 gal tanks already give diminishing return, let alone 165 gal ones?
The P-47N can use 2 x 300 gal tanks, with similar poor return, extra 25 miles under prevoiusly stated conditions.
See above for P-51H discussion. With any similar operating profile the extra 9 gallons of the P-51D capacity was meaningless, and taking advantage of superior cg envelope of the P-51H gave an advantage to the H.
Radius of bombed-up P-51 is indeed fine. The radius of a bombed-up P-47 (from 1944, so it includes -N) can be significantly boosted by using belly drop tank, the aircraft being safely under overload weight. Or, use a drop tank under one wing adn under belly, bomb under another wing. Similar with P-38, as we know from Ploesti raid (300 gal DT, 1000 lb bomb), that is before it acquires extra 4 racks under inner wing.