Between the wing and (IIRC) a fuselage stretch, the delta F-16 did not need the spine or conformals. There was lots of fuel in the wing.
From IDR, 1995...
The next step in F-16 evolution, the F-16U, addresses this problem. The F-16U is competing against the F-15E for an 80-aircraft launch order from the United Arab Emirates, and a long-delayed decision is expected before the end of this year.
The F-16U is a return in principle to the F-16XL, which General Dynamics demonstrated in 1983. In both cases, the goal is to increase the F-16's range by adding internal fuel and decreasing the drag of external weapons. The F-16U has a 1.4 m fuselage stretch and a 65 m2 cropped-delta wing. The planform, large-area leading-edge flaps and twist and camber draw on experience with the F-22 and the GD Advanced Tactical Fighter proposal of 1986. The structure (aluminium alloy ribs and spars and composite skins) is different. The wing is bigger in span and area and deeper in section than that of the XL and, together with the fuselage stretch, increases internal fuel capacity to more than 7200 kg - well over twice that of the F-16C. Wing root troughs hold four AIM-120s, and underwing hardpoints can carry four 970kg weapons or eight 450 kg weapons in addition to a pair of AIM-9s.
The F-16U would be powered by an uprated version of today's engine. In August, General Electric plans to run a 155 kN F110 demonstrator with a higher-airflow blisk fan based on F120 technology, matched to the F-16's inlet, and fitted with a scaled-up vesion of the F414's composite-lined augmentor. Pratt & Whitney may offer its F100-PW-229 Plus.
With the uprated engine, the F-16U is expected to accelerate as fast as the lighter F-16C. It will have a better sustained turn performance because of its lower wing loading, and is designed to handle better at high angle of attack.
Other potential improvements to the F-16U include GE's Multi-Axis Thrust Vectoring (MATV) nozzle, tested on an F-16 last year. MATV has been under development since 1987, and is considered ready for transition to production. With MATV, the F-16 demonstrated advanced air-to-air manoeuvres such as a fully controlled Cobra and J-turn. (It would be ironic if the UAE's aircraft were to have the MATV, which was being jointly developed by GE, GD and Israel before releasability concerns forced Israel out of the program.)
Further in the future, GE is looking at a 180 kN F110 with the core of the CFM56-7 (a new version of its best-selling commercial powerplant) and some of the advanced variable-cycle features of the YF120 ATF candidate engine. With this powerplant, the delta F-16 could supercruise with air-to-air weapons.
With a single 180 kN engine, long range, multiple sensors and advanced avionics, the ultimate F-16 starts to look a lot like the Air Force's version of JAST. "You have hit that right on the head," comments one non-Lockheed observer. "Do you want to replace the F-16 with something that goes Mach 2, does air-to-air and air-to-ground [the F-16U] or with something that doesn't have that level of performance and carries a lower payload?"
The only attributes of JAST that the F-16 cannot be modified to emulate are all-aspect VLO and STOVL, only one of which is of interest to the USAF. Unsurprisingly, Lockheed models and impressions depict F-16 growth versions with larger, stealth-contoured inlets and low-observable "chevron" edges to doors and other apertures.