The Start of this thread was "What if the USAAC, recognizing the historically expeditionary nature of the US military, and the need to reach out to the enemy rather that wait for the enemy to come to you, had specified "P-51 like" ranges in fighters such as the P-40 and P-47?"
Now maybe I misinterpreted it But I took that to mean 'what if the USAAC had specified the longer range either at the beginning of the P-47 development or in the earliest stages of the P-40s development'. Changing specification too much in the middle of production leads to changes in tooling and lost production that many countries were hesitant to do.
I do not have the full specifications or requirements that the P-47 was designed to. These would include not only top speed, ceiling, desired climb rate or time to altitude, range and armament but things like landing speed and field length "G" loading strength and landing "G" load requirements among others. Some of this can be traded back and forth on paper while the plane is in the proposal stage before contracts are signed with performance guarantees. Curtiss lost over $14,000 in penalties on the XP-46 contract because the prototype would not perform up to contract specifications. The Army may relax some of these requirements once a plane is in service, like adding the rear fuselage tanks to the P-51, rather than call for a new design, but while a plane is still on paper or in prototype trials they are going to want it to meet the specifications (or mutually agreed modifications).
Adding hundreds of pounds of tanks and fuel, even if you can find room, will impact not only field length and climb, but those stress requirements.
If we are talking about specifying the extra range/fuel from the beginning, I am pointing out that the power plant (engine and propeller) available or planned was not capable of the performance not only of the real the P-47N engine and propeller of late 1944/45 but even of the field modified "D"s of the end of 1943.
Once again for an Idea of the performance "hit" the plane would take see this chart:
http://www.zenoswarbirdvideos.com/Images/P-47/47TOCL.gif
Look at the climb rates for the different weights. Please look at the notes, like this is for O degrees Celsius and add 10% to climb times for each 10 degrees above O degrees. Also note that after 5 minutes at military power the engine is throttled back to "normal" power which is 1625hp instead of 2000hp so a lot of the climb figures are not true combat figures but you can still get an idea of the performance "hit" of the extra weight.
Drop tanks helped designers out because they could increase range while not hurting performance at short range like a bigger airframe/ heavier internal tanks would. But the planes "Combat radius" is limited to how far the plane can fly after dropping the tanks, engaging in combat and then flying home with enough reserves to handle weather conditions and minor navigation errors.
Please note that the later P-47Ds had 15-25% more power available than the "B" and "C" versions to handle not only the drop tanks but the change from 305 to 370 gals of internal fuel. P-47s sometimes used their water injection for take-off while carrying heavy loads from short airstrips in hot conditions.
When building fighter aircraft it helps if the "balance" is not tipped too far to one side or the other. A long range fighter for escort use is great, as long as it can actually fight once it gets there or can fight near it's own home base if the situation changes.