If you use a 1U or Flex PSU with this setup, you can even remove its 40mm fans if you wanted. Damn that is a great idea for a setup.
Quickly took @EdZ idea into sketchup, also thanks to @iFreilicht I remembered there are 1U PSUs!
I took the dimensions of the FSP 1U 700w Platinum as it seemed very reasonable in size and power output!
92mm fans seemed like the biggest coherent fan size you could go with without sacrificing either on cooling or size. 2x bottom 120mm fans with a top 80mm seems like it would work as well!
Other than that, going with the 700w 1U PSU on my layout would be possible, opening up to more power hungry components.
The volume of both these cases would be very similar actualy, with EdZ coming in at the slight lead.
I took the dimensions of the FSP 1U 700w Platinum as it seemed very reasonable in size and power output!
92mm fans seemed like the biggest coherent fan size you could go with without sacrificing either on cooling or size. 2x bottom 120mm fans with a top 80mm seems like it would work as well!
Other than that, going with the 700w 1U PSU on my layout would be possible, opening up to more power hungry components.
The volume of both these cases would be very similar actualy, with EdZ coming in at the slight lead.
Hm, what about putting the PSU between the mainboard and GPU or behind the mainboard? That way the whole case gets wider, so you could use two 120 or even 140mm fans, which should reduce noise.
The width actually isn't what's the limiting factor for 120mm here, the height is..
So increasing the width wouldn't offer much benefit in my opinion.
Ah that's what that line meant, sorry! Well in that case, which of the two options has the higher surface area? It seems sensible to have an 80mm up top because the PSU needs less airflow than the other components and use the real estate you get from that to increase airflow and pressure towards the bottom with two 120mm fans.
No Worries! 
The 120mm route would have the best capacity to cool off the system due to how you'd be able to direct and really maximize most of the airflow to where you need it, a shroud is still an important part of the cooling system here and it would be a pain to create one with the 3x 92mm fans.
However, as you mentioned with the PSU sandwiched in between the bottom GPU and Mobo, that could actually be a viable option if you really want to shave off some height.
Cooling capacity would be decreased ofc, prototyping would become more complicated but all in all... you could place it horizontally, get a symmetric looking case at the front, quite well balanced HTPC with a total volume taking a stab at the Node 202 and winning.
The 120mm route would have the best capacity to cool off the system due to how you'd be able to direct and really maximize most of the airflow to where you need it, a shroud is still an important part of the cooling system here and it would be a pain to create one with the 3x 92mm fans.
However, as you mentioned with the PSU sandwiched in between the bottom GPU and Mobo, that could actually be a viable option if you really want to shave off some height.
Cooling capacity would be decreased ofc, prototyping would become more complicated but all in all... you could place it horizontally, get a symmetric looking case at the front, quite well balanced HTPC with a total volume taking a stab at the Node 202 and winning.
The question now is of course how to decide which option to use. They both have drawbacks and advantages. From experience I would say, especially if you have never done this before, use the option that is easier to prototype first, just to get a feel for everything.
To be frank, the first one but with the FSP 700w PSU is the most attractive one to go for in my eyes, a lot due to the fun in experimenting with airflow and waste heat from GPUs, setting a boundary where the cooling system will play a big role, pushing me to test out new and fresh ideas that I can use outside of PC components!
