I'm putting this out there because I couldn't find the answer, and took it upon myself to just wing it...
If you're unlucky enough that your 68 switch won't turn over anymore due to the tumblers being stuck or just wore down to the point where the key won't turn it, and you don't want to be paying the Corvette tax (I have another name for it, but it ain't pretty lol), and you don't really mind not being "genuinely NCRS omg le gasp what?" correct, then here's the low cost solution that worked (by the way I was changing my dash anyway, drilling the lock resulted in nothing good, still stuck but I digress)
I bought the following:
AC Delco D1415B switch and D1400B lock cylinder right off Amazon for a total of 24.99
a 1967 Camaro chrome bazel for 12 bucks
Grand total of 37$ which isn't even close the cost of just a 68 vette ign switch (45 bucks for just the switch, the keys are another 25 and the nut is another 25 and then god forbid you need the ign switch plastic indicator which is another 31 bucks...)
90$ saving total (that's a lot of beers for the shop!)

the 68 ign plug will go right into it, the only "problem" which isn't a problem at all really, is the plastic retaining prongs are clocked differently (who cares, the connector won't come loose, it goes on there pretty tight as is)

Far as I'm concern it looks great...I may 3D print that plastic ign plate for it...

 

Good score!!

 

Love it! Thanks, mirage

 

I was able to safely remove ign decorative ring and added it to the new set up, fits like a glove

 

Thanks. Good job. Do you have a picture of how it looks on the backside?

 

is this what you're wanting to see? I'm pretty much all done wiring the car up so the ign plug is on the car at the moment, but those square cavities on the outside is what won't line up to the plug's prong. The spade connectors line up and are all at the same location as the factory switch was, so no issue there.