Mark, which "M series" truck do you have?
If you have an M35A2, there is overdrive gear/kit that a member here makes and it will get you the highway speed you desire without compromising low end torque. The cost of this is relatively cheap when you consider your other options.
If you have an M35A3, axle gears or a transmission swap would be the way to go. Unfortunately, to my knowledge, no one has found a compatible automatic transmission that will fit. If they have then they haven't shared it on this site. Some have talked about mating the Spicer manual transmission in the A2 to the Cat 3116 in the A3 but I don't think anyone has actually done it.
Check out this thread for some useful information:
Regardless of which truck you have, larger diameter tires will aid in gaining top speed. However that can be relatively pricey when you consider the nominal increase that will get you. That is, unless you are going from really small to really large tires but really large tires require either extended dog bones, bobbing, or bobbing and lifting.
Also, be careful talking about top speed on this site. Some members get really upset if they hear that you want to cruise at more than 55mph.
Welcome to the site!