If you put an 8x10 lith negative with very fine type on the edges, enlarge to 20x24 and use both 240 and 300 mm lens , my money would be on the 300 being sharper and readable.
If your workflow is portrature and edge sharpness is not critical then the 240 is totally adequate.
For my work flow *which is probably different than others* I would never put the 240mm lens on my enlarger, I need critical sharpness on the edges therefore my experience dictates the 300 and even 360mm for 8x10 work.
So I guess the question is: Can I use a 300mm on the beseler mcrx/aristo t-12 conversion? I hope so! I do landscapes and the whole reason to go to 8x10 vs my 4x5 is for sharper/higher resolution prints.