Custom build of a Kenworth C509 as autralian roadtrain

[frizzen]

That's always the tricky part!

Sometimes you can find a manufacturer or upfitter that has scaled drawings or blueprints posted somewhere on their site.
Sometimes you can do an image search of something like "ambulance drawings" or "ambulance blueprint" ''ambulance vector"
Other times ya just find some inspiration pictures you like, and make it up as you go. It's your world, madbe it needs some happy clouds

In RC customizing, rather than getting too hung up on this MUST be exactly 1/16 or 1/14 scale (or 1/13.5) it's usually a lot easier to start with picking a tire&wheel or a whole donor vehicle that looks or acts about right. Then adjust scale of the entire build to the wheels, or track width, or whatever makes sence with what you have available.

Nobody will get too uptight about the 1/10 Suzuki Samurai being larger than the 1/10 Chevy Suburban when they're both using the same chassis.

Start up a thread in like 'general rc' or 'body work' with a range of what you're looking for, somebody might be able to help ya track it down?