LJ
Correct! The RJ is based upon the Boeing Model 717 (not to be confused with the Boeing 717) and the E-3D is based upon the Boeing 707-320. Here's the difference:
This military version of the Model 367-80 is identified as the Boeing Model 717: it differs primarily from the later Model 707 by having a smaller-diameter fuselage, deletion of cabin windows, reduced size and weight, and accommodation for 80 passengers or an equivalent weight of cargo on the main deck.
So to use the ONE E-3D in mothballs would indeed take megabucks and a complete redesign of the RJ's design drawings.
iRaven