It would seem that the 24MP DSLR image will print at 302 dpi, just in excess of the 300 dpi limit of the Chromira. The scanned film image will print at 566 dpi, well in excess of the 300 dpi limit of the Chromira. My question is what happens to the "excess" dpi; put another way, once over the 300 dpi limit of the printer, do the prints look any different?
If a client insist on an exact size...
If you have control, try to print at your printer's optimum (or maximum) ppi...
You could print the 24 mpx at 300 ppi at slightly less than 20 * 16, or leave a border.
Scans can be expensive, but with some scanners you can scan at exactly the pixel dimensions you want - of course you could do this optically if you were re-photographing.
A good printer driver will make quite a good job of resampling the image to the required output ppi - you could try re-sampling in photoshop to see if the results are any different.
If you have 566 scanner pixels per print inch, and you print at 300, I expect the results to be worse than if you scanned at 300.
Could you print so that you have 600 scanner pixels per print inch? ...even if you could only print at 300, it would be an easier re-sampling operation that would look better in the end result.
Alternatively, you could find someone with an Epson, and print @ 360.
...but most photographers do not care (and are not concerned about marginal differences in quality) and most customers would not notice.
One of the reasons I will be upgrading my camera is that if will enable me to fill the width of my Epson printer @ 360 original camera pixels per print inch.