What Thread Size For Reverse Mount Adapter, Canon EOS + Nikkor 50mm F2.8

Yes, 40.5mm. You will need a reversing ring to mount on your bellows. There are options available, but not easy to find.

The 50mm f2.8 EL-Nikkor is designed for projection ratios of 2x...20x. The 35mm film is placed on its "mount" end, while the "filter" end points toward the print film. The image on the print film is between 70mm and 700mm across. If you use the lens for "taking" rather than enlarging, and mount the lens "normally", then it is good for magnification ratios of 1:20 up to 1:2. This means it is great for larger coins and medals, slabs, albums, etc but it is not optimum for small coins, higher magnification closeups, etc.

But if you reverse the lens, its characteristics are reversed. Now the lens is optimum for magnifications of from 2x...20x (!!). In reality, the f2.8 limit makes it good up to about 4x, but that still gives you from 2x...4x magnification range.

Note the region from 1:2 to 2:1 is a "hole" that most enlarging lenses don't do as well in. This is where duplication lenses come into play. They are specifically optimized around 1:1 magnification, and are usually good from 1:2 to 2:1.

Both the Omegaron lenses are decent quality enlarger lenses. Stop each down to f8 and you'll get high quality images from them.

The two Nikkor lenses are probably "OK" reversed at magnifications from 2:1 up to around 4:1. Beyond that the performance will drop off rapidly.

Your Vivitar bellows is probably a T-Mount, so not M42. "M42" means 42mm diameter, 1mm pitch. "T-Mount" is 42mm diameter, 0.75mm pitch. Sort of compatible but not really. So pretty much any brand or diameter of reversal ring will work...T-Mount to 49mm, 52mm, etc. Then a 40.5mm step ring. 52mm seem to be the most common.