During the second pilot MJ might not have given any thought to what the Valiant
looked like if it wasn't called for in the script. That could have been different if they had considered putting an image of the ship on one of Spock's overhead displays (something that could have been done in TOS-R).
The only tiny visual clue we have about that era in TOS is the DY100 from "Space Seed." That design supposedly predates the Valiant
by at least fifty years and maybe even seventy years. The DY100 design is obviously way more advanced than any real spacecraft we've ever had even presently. That speaks of space programs that didn't get derailed after Apollo.
So, to me, that means the Valiant
shouldn't look too much projected from what we have now. Rather I'd look at references like The Spaceship Handbook
and The Saucer Fleet
as well as other similar references and try to springboard a design from there. This material looks at the fictional and proposed designs for spacecraft from the 1920s to 1960s. The key difference, of course, is your design is meant to be an interstellar rather than strictly in-system vehicle and so you adapt with that in mind.
You could also look at some of MJ's early sketches when he was trying to get a handle on a direction for the Enterprise. However, I don't know how much of that material survives or is available. I think I have seen some things online that weren't included in the Star Trek Sketchbook.
I'd also be inclined to ignore most if not all references from post TOS in terms of distancing yourself from more contemporary Trek influences (particularly ENT). I think that's a more likely path to work out a design that looks and feels more consistent with TOS' style.
My two cents.
Oh, for what it's worth here's my offhand conception of the Valiant
from some years ago.
I essentially took the hull of the spaceship Luna
from the 1950 film Destination: Moon
and started playing with it in Photoshop. At the time I didn't think too much about it beyond that. My basic idea was the Valiant
was initially a fast relativistic ship that was fitted with an early form of space warp drive. I like the name used in the Trek novel Federation where it's referred to as a superimpeller.
My thinking was that in the early years of interstellar starflight different configurations were experimented with to find an optimal design rather than assuming two nacelles were always there from the beginning and throughout until the (Trek) present.