You're not likely to get any distortion on a modern 300mm on 4x5. But if you are really paranoid about it, select a lens like an Apo-Ronar, G-Claron, or Fuji A. But I'll bet the Nikkor M and Fuji C will be just as good. Ordinary 300 or 305 plastmats will be cheaper on the used market, but a lot heavier, and probably have more vibration due to big #3 shutters, as well as the extra stress on the front standard.
I've never owned a large format lens with barrel distortion.
If the 300M Nikkor is anything like the 450M Nikkor, it will be distortionless throughout the range - I don't get any with my 450M even in my 14x17 negatives. With movements applied.