This makes it a little easier to make suggestions, now that you have defined the performance envelop.
The Canadians used a 251 (265?) c.i. engine in their trucks with a different transmission. I believe they had a higher top speed.
The real problem you face is not the engine but the rest of the running gear. These trucks were built to keep pace with 1950's tanks. Tanks didn't go very fast back then.
There are 4.89 conversions for the front and rear end. The transfer case can be a problem because at higher speeds it gets very hot. Maybe a transfer case cooler with fan will help with that problem, but it is just not designed for sustained high speeds.
There are also disc brake conversion kits available and there are power steering conversion kits available also for control at higher (freeway) speeds.
The point of all of this is not to dissuade you, but to be sure you have considered all the additional engineering issues (and COSTS) you will be facing if you modernize the truck.
You might want to consider discussing this project with some of the people over on G741. All they focus on are M37 & M43 vehicles over there. There are some very knowledgeable power train engineers, machinists and experienced hobbyist there who can help you with your projects. It would be worth your time (IMO) to talk to them.
Good Luck with your project!