Hi @Rob Leese
I agree with you that FW&D’s names Silver CHIEF & Silver WARRIOR are wonderful E5 names which evoke imagery of “Everywhere West”.
Although TZ decals in O scale are not commercially available, Microscale does have an HO set of CB&Q E5 decals with all of the names & numbers and the Texas Zephyr lettering (set 87-581).
Microscale also makes decal paper for use with laser printers.
So if you are decal inclined, you could make an enlarged copy of the HO decals onto decal paper .
(1/48) / (1/87) = (87/48) = 1.81 Scale Factor
I am an out-of-the-Box guy and have no direct experience with application of decals onto models. I would probably damage my model in the process & end up with a Frankenstein’s E5 !!! Silver MONSTER !!!