The Capella-class utility ship could attach either nacelles or other equipment to ventral and dorsal hard points as needed for various missions. This ship is a recent addition to my website, the Starfleet Museum.
Somewhere I have the old FASA model kit of a transport tug as shown in the Starfleet Technical Manual, from back in the 20th century. Looking at your work makes me want to find it, and try to build it up.
I never liked the idea of movie-era ships "un-refitted" to TOS style. Seemed kind of lazy. So I figured Starfleet would approach the layout and role of later ships by upgrading earlier ships. The "planetary sensor" pod is a backdated Oberth pod and the nacelles are from a STIII (or III) pre-production sketch (as is the fighter hanger thing).