Walter,
Does anyone still make the style of decal that you refer to? I wouldn't even attempt to use this one. Would the vinyl door graphics that FastSigns.com sells work as an option?
Ruby is long gone, and the only water slides I'm aware of are now computer printed on a stock that supposedly is equal to the old material.
I can't say for certain because I haven't held any of them in my hands.
Water slide to my understanding was a process originated in France in the 19th century and improved until it became no longer cost competitive in the 80s. Vinyl and other materials had become a lot better and more importantly the adhesives.
In fairness I'll also say I have a fellow a couple miles from me who got into computer cut vinyl and Skotchbrite back in the 80s and makes some labels that are probably as good as waterslides in most respects. I don't like the thickness compared to water slide though. That may not make a difference in your job. I've been watching one set on glass since 90 that are still in place and in tact exhibiting quite good wear, including wind abrasion. One of his customers for years has been a garbage hauler and the product has stood up well in that application which includes pressure washing.
Vinyl stock like real Formica is a multilayer material with the top layer being similar in purpose and nature to clear coat on a vehicle. Over time (years) this layer does fog for want of a better descriptor.
The second shortcoming to computer cut vinyl is the sharp edge with no clear protection to resist water etc.
One big advantage to current vinyl is the cutter's ability to produce a large multipiece label on a single backer sheet that saves a lot of work applying it.
In 2020 I'll grudgingly say vinyl can be almost as good as a water slide.. Hey, I'm an opinionated old fart. We've got adhesives today that hold jet airliners together so sticking vinyl on a car is certainly doable.
If you want to take it to the next level I can still remember how to do gold leaf inside a window and I can probably do the shadow as well if my hands can hold the brush.
Suggestion, there may be a couple guys doing waterslides for the antique engine collectors. If there are you'll be able to locate them on a site called SmokStak.com.
Whoever you choose to deal with be absolutely sure they'll deliver water slides.