Sojourner - that isn't excess resin-flash, but the very thin nacelle extensions past the rear-housings; not painted yet, but the end-plates are auxilary impulse-engines.
Have seen the Ariel. If you compare that design with the plans for the Discovery I linked, altho the Ariel saucer looks more like a flattened discus, there's general similarities despite the several decades lying between the times pen was put to paper. (Yep, the originals are real inked plans; still hand-draw most of my work.) But the intent was spelled out in the back-story for the model, to use the original refit saucer as is; it might look a bit smallish fitted to the Discovery hull/nacelles, but not freakishly out of scale.
Publishr - agree with you not enough done in Trek canon to evolve the refit design. Seems each time the producers go to a new series/movie-arc, they just summarily dispense any more references to the primary ship-designs.
The 'muscle-car look' isn't hard to come by for me with a '69 Charger sitting only a few feet away in my garage. (NX-1969 takes some cues from my ride.)