David,
Horizontal tube mounting for the GK-1 is just fine. Works OK; this tube is specced to mount either way........
Width of the GK-1 pcb is 80mm, so you could make a slimline case down to 85mm with ease. This is just shy of 3 3/8", not too tall.
I use CAT5E for all internal signal wiring in the GK-1, and for the input signal wiring in the power amplifiers. However, I won't comment on the interconnects between the components; this is an area I'd best keep away from as everyone has their religion here and I've no wish to upset anyone. I will say, however, that I like the Ubyte design from Jon Risch, and Lewis Muratori's cable on my website is damn good, too.
Cheers,
Hugh