Ships like this have never made sense to me. Where do you fit the ship systems from the engineering hull? That's bulky equipment. Even worse, some classes just project the Nav Deflector out of the bottom of the disk. It's a high-powered forcefield projector, not a radio antenna!
In the original Connies there isn't all that much unique engineering in the Engineering Hull, so it's pretty easy to imagine that they could make an independent saucer ship like this. As for the nav deflector, that technical rationale was a late arrival. When this ship was designed, that dish was understood to be the "Main Sensor". The nav deflector array was, I recall, the three white dots that appeared on the front of the primary hull. It was either something in later movies or TNG that reassigned this device's function.
But the dish-as-deflector was then thoroughly retconned in... book authors and.Starship Creator saw to that. And even accounting for TOS's odd M/ARA configuration, a warp core and fuel storage is bulky for the saucer to contain.
The warp core was a TNG creation. Have you ever looked through the original, 70's Ballantine Enterprise blueprints? It wasn't until the Motion Picture Enterprise (several years after the design of the Scout/Destroyer) where we started to see power train integration between the two hulls.
The Classic warp reactor may not have been a 'core' in form, but it is a M/ARA power plant regardless. Those still require plasma conduits, support machinery and fuel storage that would be difficult to fit into a Constitution-class primary hull. I suspect that's the reason Miranda-class hulls are bulked up towards the stern.
Incidently, I did once see those plans when in middle school. Even then, I got the sad impression that the writers were in way over their heads and just trying to bluff their way out.
Well... thanks for clearly seeing the passion/spirit behind these pieces. Sure, playing chicken with Klingon D-7s was always kinda cool - especially in my younger days. But the REAL appeal for me has always been Trek (and all sci-fi's) exploration of the unknown... and the challenges that come with that experience. I suspect you know EXACTLY what I mean