not knowing how tall the tires are you wont be able to figure it out. so using the link grigg posted it says my 40 250 will run 2261 rpm @ 45mph and 2512 rpm @ 50 mph. that is with a 4.56 rear, 30.5" tires(7:50x16's). that's exactly what my tach in the truck read at 50 all the way to aths southbend show and back for a total of 1300 miles. lots of seat time to listen to the 270 sing. while i was there a friend of mine ran it up over 3000 rpm for a short duration. because i was too busy freaking out what he was doing to my engine i didnt get a chance to see how fast we were going while he was laughing at my drama. he assured me it could do that all day long and probably can just get used to the noise. being it a 49 the cab design will make it quieter and you shouldnt notice it as much at least thats how it is in the "Ashton". i run that at 53mph and its turning that 270 2700-2800 rpm and you can have a conversation in the cab. in the 40 no way.
id wait until you have the truck to worry about that. your driving yourself nuts. been there done that
hope that helps.
