Juicebox and SimpleViewer run on different platforms (Flash vs HTML) and have different sets of configuration options so it would be difficult to exactly replicate a SimpleViewer gallery using Juicebox.
However, the following notes might help you to get close.
I'd like to be able to make my own thumbnails
This is possible. Each thumbnail in a Juicebox gallery is assigned a unique URL so you could create your own thumbnails and point towards then using the thumbURL entries in the gallery's XML file.
and have a larger fullview image than what I've been able to get Juicebox to produce
Juicebox-Pro configuration options such as imageScaleMode and imagePadding (and rearranging other gallery elements) might help you achieve a larger main image in your gallery.
as well as the complete horizontal layout, as opposed to the fullview above the thumbnails
You can position the thumbnails in a 3 x 3 grid to the left of the main image in Juicebox-Pro (though not in Juicebox-Lite, the free version) by setting thumbsPosition="LEFT", maxThumbColumns="3" and maxThumbRows="3" in your gallery's XML file.
Here is a demo Juicebox-Pro gallery which closely resembles a SimpleViewer gallery.
Other demo galleries which demonstrate what can be done with Juicebox-Lite and Juicebox-Pro can be found here.
Also, testing Juicebox in WordPress hasn't really given great results (mostly because of the issues above), so I guess I need to know if there's some sort of optimal WordPress layout to use Juicebox with in order to do something like what I had in Simpleviewer?
Many WordPress themes use a central column to contain the content with large borders to the left and right sides.
With the usable width of the page reduced as it often is, the default Juicebox layout (thumbnails below the main image) is likely to be the optimal layout for most WordPress users. If you were to position the thumbnails to the left of the main image, there may be little space remaining for the main image to be displayed and it might appear very small. If doing so, I would recommend that you reduce the size of your WordPress theme's borders to accommodate such a Juicebox layout.