I don't believe they ever managed to stick a fan engine on a B707, the wings are significantly closer to the ground ...
The E3A Sentry is effectively a CFM 56 powered 707. A commercial version, to be known as 707-700 was also flown in prototype form but not developed further. The P&W JT3D and RR Conway were of course fan engines too but with low by-pass ratios compared to today's turbofans. If you look at those machines on ground I don't think CFM56 or equivalent would have needed major modification, perhaps shorter pylons.
IIRC the problem with fan engined 737 requiring the oval nacelle arises because the 73 was designed with low ground clearance to simplify loading/handling at remote/ill-equipped outstations.