It all depends on what you're talking about.
There are two fan-design "refit era" dreadnoughts which were created back in the 1980s... the Ascension class (based upon the Belknap... essentially a "light Dreadnought") and the Federation Class (refit). I think you're talking about the Federation(r).
If you're talking about some other version (ie, one of the Jackill's variants) you can disregard all of this. But there was a set of VERY nicely done drawings of the Fed(r) done by StarStation Aurora way back when. I used those as my reference for a work-in-progress model I've only recently started revisiting.
Here is my Federation-class dreadnought, alongside an unpainted, out-of-the-box AMT kit. The surface finish of my parts is not perfect yet, but the shapes are nearly exact at this point.
First off... here's a comparison of the stock AMT secondary hull to the Federation(r)'s secondary hull. Note the following:
1) The secondary hull is significantly longer overall.
2) The hangar deck outboard platform is larger.
3) There is no Arboretum.
4) The layout of windows, gridlines, and docking ports is different.
5) The dorsal is significantly larger (my part here is VERY crude right now).
6) There are both forward and aft firing torpedo tubes.
There's another major difference that most people overlook. The B/C deck superstructure should be enlarged. Here's a stock AMT kit B/C-deck superstructure:
and here is the enlarged Dreadnought version:
The idea is that the secondary hull internal configuration is significantly different... mainly due to the presence of twin matter/antimatter reactors instead of a single one in the Heavy Cruiser. The Federation(r) has twice the power output of the Enterprise, in other words. It pays for this by giving up certain amenities (like the arboretum). The hangar space is larger, and supports both shuttles and combat craft.
The primary hull has a different bridge module with two docking ports instead of just one. However, the bridge itself is just about the same. The main difference there lays in B and C decks, which are set up as Fleet Command Operations facilities. The Dreadnought is typically the flagship of a fleet, and is commanded by an Admiral in the Fleet Operations Center, while the ship's captain runs the ship itself from the Bridge. This is why the B/C deck section is larger... just more stuff going on there.
The other differences are better known... the engine nacelles on the secondary hull are on a flat, "wing-like" pylon rather than on twin angled pylons. The nacelle on the dorsal is different, however, and most people get this one wrong. If you're modeling it, you need to make it from two INSIDE nacelle halves, not two OUTSIDE halves. Of course, then you'll have to add intercooler fins, but better than than have the wrong "engine grill" detail.
It's also worth noting that the positions of the "control reactor" ribs on the underside of the nacelles differs on the Federation(r) from that seen on the Enterprise.
Anyway, none of this is real so you can make it however you want... but if you're trying to do a Federation Class (refit) per the Fandom-created design, these are well-established by the folks who did the original designs.