If only it were that simple. There IS NO GR7 kit in 1/24. The Airfix GR7 is 1/72.
The GR3 was a modified GR1
The GR7 is an almost completely new airframe, with new nose, new bulging canopy, new instruments, larger intakes with double auxilliary doors, longer front nozzles, bigger wings...
To see the work involved, get hold of the 1/72 GR3 and GR7 kits and compare the airframes. THEN work out how to build everything 300% larger.
Gentlemen! You can't fight in here; this is the WAR ROOM!