Just thought of this. Why not an F-5, etc with extended wings? Poor mans U-2? Might need different engines at some point along the line.
If you are going for high altitude, keeping the existing pure jet engines makes sense. As for the extended wings, I can see two approaches, reducing the sweep of both the leading and trailing edges and going for a longer wingspan until you reach the same wingtip chord or add-on extensions that fit in place of the Sidewinder rails or tip tanks and extend the wing, either keeping the same sweep for leading and trailing edges out to a very narrow tip or keeping the trailing edge sweep and reducing the leading edge sweep out to the same narrow tip (somewhat like the later J-7 wings).
