Not sure how much more total effort it is than to redesign the entire arse end of a Phantom including the thrust structure.
Noting that the frame is structurally critical and quite expensive, to be sure, and that associated design analysis needed to prove the part and the associated structural integrity, but, other than the frame redesign, there would have had to be a significant effort to re-stress analyse the lower Phantom fuselage and mounts, rejig systems etc. etc. etc. to handle the quite different Spey. I mean were not talking cheap here under any circumstances, but I'm not sure that the total effort expended would increase much more than 10% of the total job to manage the redesign of the tail frame. Re-engining a Mach 2 airframe is never an inexpensive proposition. Your structural and flight test and recertification efforts frequently swamp many hardware redesign costs in an aerospace program.
Granting that it would be more expensive, though, it certainly wouldn't be impossible if one wanted to make the switch.
Start with a A-5B, delete the internal bomb bay in trade for additional fuel and avionics (imagine the ECM suite you could carry! Vigilante Wild Weasel anyone?), add an additional wing stores point each side (for 6) and strengthen the centreline station. Plumb the outermost stations for AAMRAM and Sidewinder. Optimise radar for surface search and weapon guidance and an external IRST ball.
Pretty killer long range naval strike platform, no?
Paul