The 416 Ruger's looking to be a nifty cartridge, but I don't like the Ruger's tiny three position safety and would like a longer barrel.

So, I have a Model 70 classic action in 7mm STW, which I've decided not to convert to 404 Jeffrey (I've ordered a CZ 550 instead ), so would the 416 Ruger be a feasible conversion on that action? Are reamers available?

Yes indeed!
I think like you and would like to do the same thing.
In fact my 300RUM M70 Classic feeds my .416 Ruger dummies like poop through a goose.
The non-windowed box on my .416RemMag M70 Classic feeds well too.
Both work fine.
Your 7mmSTW box should do the trick.
Might need a little feed ramp and rail polishing if anything.

The longer box on the M70 will allow you to have greater net case capacity and out-do a .416 RemMag with the .416 Ruger.