Never a fan of any V-8 diesel, too much vibration too little lugging ability. Had a 3208, didn't care for it. How hard is your 18 working? Guess, I'd need more info on what you consider working hard. Also, Is every thing up to snuff.....injectors, fuel pump, oil pressure, turbo?
If nothing more than heavy field work....2255 or 2-135/155
If needing tractor for other lighter duty jobs, you'd be better off with what you have or a 2-110 ect.
We pull a 13' Hesston moco with a 1755 in 3rd over and rarely shift down due to lack of horses.