Although the Phantom was unquestionably Phabulous, here would be little value to the USN in developing a Super Phantom using Spey. Although its higher thrust did benefit the F-4 down low, the increased drag at medium and high altitudes negated the gains and the J79 Phantoms actually outperformed the Spey ones in those situations. Plus, you're adding a whole new engine not used anywhere else in the US to the supportability and logistics chain.
The A-7D used the Allison TF41, an Americanized Spey. Are you thinking more along the lines of a Strike Phantom procurement leading to no A-7D?
Just adding thrust without updating other systems, improving maintainability (there was a reason that one of its nicknames was "The Beast")and making aerodynamic and fuselage changes to take advantage of the thrust would mean you'd end up with simply a noisier Phantom. Making those changes would be very expensive.
I was thinking the F-4K design could be used as the basis for the design. Also, modernizing an existing design tends to be cheaper and quicker to develop than a fresh sheet design, and allows some existing machinery and competencies to be reused. I don't know how difficult it would be to improve maintainability, but I think cockpit ergonomics might not be as difficult to improve. McDonnell Douglas could use research developed for the F-15 program to assist in that effort.
Not to be overlooked is the reality that some members of Congress would use work on a Super Phantom as a rationale for killing the far superior F-14.
I suppose that could be a risk. After all, the Phantom started as a fighter/interceptor and only later was moved into more strike oriented roles as newer fighter/interceptors entered service. I don't know if the Phantom could be modified to use the AWG-9/Phoenix combination though, although I think the F-4L would was proposed as a Spey powered AWG-9/Phoenix missile variant.
USAF faces a similar situation; they didn't want too much more development going into the F-4 lest that be used as an excuse to kill the F-15.
The F-15 is far more maneuverable than the F-4, although perhaps the USAF could push for an improved F-15 baseline design featuring canards and 2D thrust vectoring. The Super/Strike Phantom could lead to a better Eagle.
As an alternative to the Congressionally imposed Hornet, a better and probably cheaper choice would have been to continue with the F-14 into the original F-14B & C series (If they could have gotten the F401 to work) and move forward with the A-7X, the latter aircraft discussed elsewhere in this forum.
Reading up on the F-14, it looks like the program was derailed for about twenty years. The F-14D sounds like what the F-14B/C were supposed to be sometime in the 1970s. I didn't know that the TF30 was only supposed to be an interim engine, but it explains why something that posed so many headaches during the F-111 program was selected for the F-14. It was only intended as an early/pre-production engine.
The A-7X is quite interesting too, but I haven't been able to find much about its avionics fit and how it would have performed in aerial combat.
As far as exports go, of the aircraft listed there was no export interest, except for a country with Australia's needs, for the F-111.
It's rumored that the Shah inquired about purchasing the F-111 sometime in the late 1960s/early 1970s, but was told to ask again later due to the problems the program was having. Of course, there was also the British order to replace the cancelled TSR-2 that was itself canceled.
It wasn't that other nations couldn't afford the F-14, it's that except for Iran (where we wanted the Shah's cash to overcome a funding shortfall early in the Tomcat program), we wouldn't allow any other nation to buy the F-14.
Canada, Germany, and Japan gave the F-14 serious consideration
in the 1970s. The United Kingdom probably could have purchased the F-14 as well, although the Panavia Tornado was a vital program for the British aerospace industry.
In any case, it would become an issue of cost vs. benefit for vastly reworking the F-4. A princely sum would be needed to produce a true Super Phantom and who would fund that? And of course Dassault was out there with a whole family of modern Mirages which might be cheaper to buy and operate than a vastly updated Phantom.
Boeing partnered with Pratt for a Super Phantom powered by two PW1120s (which would have been a much better choice than the Spey), but abandoned it early in the program when they saw what it would cost and how small the market would be. Similarly, Israel built and flew a prototype of their Super Phantom 2000 using the PW1120 but abandoned it because of the cost, a withdrawl of support by MDD and the fact that they could buy F15s and -16s which gave them more bang for the buck.
That's why I was thinking more late 1960s/early 1970s for the program. While the historical Super Phantom proposals were interesting, the airframes were beginning to show their age by the time the 1980s proposals were made, and they would have been more along the lines of interim options.
The last American F-4 was produced in 1979 though, so it's more unusual that there was essentially a design freeze on the Phantom during a period of rapid development in avionics, engines, and other technologies. McDonnell Douglas might have been able to sale new aircraft and/or upgrade packages if it had developed them as an official manufacturer upgrade in the 1970s.