According to the 50tj Anniversary history of Dassault, the Mirage IVR was a recce and conventional strike variant with all nuclear capability removed. It apparently was proposed to Australia and Germany. The Germans wanted enhanced fuel tankage and, for commonality with their fighter fleet, J79 engines. Stretching the fuselage and adding the cooling scoops for the J79s (IIRC, they run hotter than the ATAR 9K engines) would make a nice whif, especially if other countries operating J79-powered aircraft took an interest.
Flying an aircraft designed for ATAR 9s on J-79s would have been sporty, indeed. The J-79 has 50% more thrust on burner than the ATAR 9. In fact, the J-79 has almost as much thrust "dry" as the ATAR does on full reheat.
If the German government had been willing to accept a small max speed decrease, they probably could have gotten a supercruising Mirage IV with far greater range if they used J-79s but completely deleted the afterburner cans and either cut the fuselage shorter to reduce form drag and/or increased tankgage and moved the engines back and shortened the inlet ducts to suit.
Imagine a Mirage IV with a, say, Mach 1.9 sustained speed (instead of 2.2) cruising at that speed for the entire range of 1000 miles (instead of 775 mi for the regular Mirage IV). Much more effective and harder to intercept in the mid 60s to 80s period than a subsonic cruise plus 5 minutes at M2.2 in and out of the target.
Using a dry thrust Spey would have been even better for range, but would have needed deeper fuselage for the larger engines and larger inlets, possibly costing a bit more speed at the top end, but likely permitting that speed to be maintained for even longer given the much greater economy of a turbofan.
Smells like very WHIFable concepts...