Jacobs does not offer a "Jake brake" for the multifuel engine. If there was room for one under the hood, and if you found a way to install one, it would be awfully loud with the original military exhaust.
I had a Pacbrake on a 33,000 pound vehicle years ago. It had the butterfly design and could be activated with a foot control, or with an electric switch. It worked flawlessly and was very quiet.
I don't know anyone who has a Pacbrake on a Deuce but it would help slow the truck down, provided the driver learns how to shift properly without overrevving the engine. That's where I see the problem. The M35A2 with it's manual trannie is not a prime candidate for a Pacbrake, or any other brake with the butterfly design.
My advice is, drive defensively and go slow and leave sufficient space between your truck and the guy in front of you. These trucks weren't designed to go fast. Towards the end of WW II, when REO started to build them, the world had not as many highways as we have today. The top speed of 55 with the truck loaded up to capacity was actually pretty respectable back then. They built trucks that could go where there were no roads.