Excerpt from Flying Wings and Radical Things by Tony Chong
"N-348/F/A18 Stol Demo Research Aircraft
As part of the ramping up for the new series of advanced fighters to come, the Air Force began exploring multiple technologies for possible incorporation into future designs. High on the list were various forms of improving STOL capabilities, as it was anticipated that NATO fixed runways would be the targets of an enemy strike. One method that seemed particularly promising was a 2D exhaust nozzle. with which thrust could be directed up or down to make an aircraft more agile or to allow for shorter takeoff runs and, with the help of thrust reversers, shorter landings.
The Air Force initiated a STOL fighter technology program aimed at retrofitting 2D nozzles to a current frontline aircraft for flight testing. Northrop proposed using a Navy/McDonnell Douglas F/A-18 Hornet for the demonstrator aircraft under project number N-346. Changes to the Hornet for this project were primarily confined to the aft end, where circular exhausts were replaced by rectangular ones. That section was already part of Northrop's work share on the F/A-18.
Because the changes were minimal, all gross dimensions would likely remain the same, with the Hornet having a length of 56 feet and a span of 37 feet 6 inches. Also unlikely to change were the F/A-18's engines: a pair of General Electric F404-GE-400 turbofans generating 10,600 pounds dry and 16,000 pounds wet thrust.
The Air Force passed on the Northrop proposal and chose to convert a McDonnell Douglas TF-15 (later F-15B) for the tests instead."
Note: the Northrop Report was NB-83-99 Proposal for Predesign Investigation of a Supersonic Survivable Fighter Technology Demonstrator March '83.
Peter Wooler was the Project Engineer; my AD people did the drawings and we had the Northrop Model Shop make the model based on a 1/32 scale plastic kit. It is possible that the actual proposal to the Air Force could be requested under FOI and that would obviously contain all available information.