I have a pair of Gassers. The yellowish truck with the white top at the left of my avatar picture is the 1953 parts truck. The all USMC green truck second from the right in my picture is the fully redone and running 1952 Gasser. Search for my post about Gassers and you will see most of my build from 3 years ago.
These engines need to be run and run hard to perform correctly. At least that has been my experience. I can fire it up and get down my driveway and dirt road while it warms up. Get onto a paved road and it is gutless. The shift to second is pretty fast, maybe 2-3 seconds with full throttle from a standing stop until I have to shift. But that time gets longer and longer and longer as I go through the gears. Pretty much, 5th doesn't get used because it can't pull any more than about 35 in it.
However, once it has been at whatever speed it thinks is top speed for the day a few miles, it will start to creep up. Better yet, go down a long hill at WOT. If you can be above 50 at the bottom of the hill, hold it there and 60 will be passed pretty soon. Once mine gets to 60 and held there a mile or so, it turns into a hot rod.
It will out accelerate the Whistlers M35, pull hills better than the whistler and run off and leave the whistler on level ground. Exhaust manifold glowing cherry red the entire time. Just needs to be blown out for the right performance.
I took a M105 trailer 20 miles last year behind the Gasser M35 to pick up some gravel for the drive way. It was mostly down hill to get there and I kept it around 48-52 mph but had to push going up hills to keep that speed. I told the guy at the quarry to fill it up. I figured I would run out of space before I overloaded the truck. I didn't know anything about gravel.
8.8 tons of rock later, we started for home and I couldn't get over 30 and out of 3rd gear. some really long up hill pulls against the governor in 3rd later to wake up the OA331 and I was having to back off on the flat sections to keep below 50. I was also worried about how fast my fuel gauge was dropping too. I think 3-4 mpg was all I got that run compared to my normal 5 mpg.
REO designed the M34/M35 truck around the OA331. It is the heart of the design and matches up with the transmission ratios and axle ratios perfectly because they were all designed to compliment and make the engine work like it was designed.
The only complaints I have are high speed long distance travel in the summer. 48 indicated with my 11.00-20 tires is all I can do in 100 degree heat before the temp gauge starts going up and up. Removing the hood side panels helps a lot, but slowing down is the only real fix. The truck was not designed to run 52 mph all day long in the heat. It was made to haul 2.5 tons of stuff cross country below 30 mph. It does that perfectly. I use it around my property all the time for pulling trees. The 425 rpm idle makes 1st low so slow and torque multiplied that some trees pull out before I even know the chain got tight. Low speed grunt is what this truck excells at.