I love those older style G-Wagons. Isn't yours a naturally aspirated engine? I don't recall seeing that they put turbos on your generation of chassis.
I've seen some people out there swap in turbo 617s. There are special elements within the engine that the NA lacks such as piston oil coolers. If you're going to upgrade your pump, I would go the full monty and get a rebuilt om617 with a turbo on it. No sense upgrading the IP of a naturally aspirated engine without giving it more air.
Probably not the suggestion you'll want to hear, but the later style OM 606.964 engine from the 1996+ G-Wagons gets around 20 mpg which I believe is astounding given the brick like qualities of your truck.
Since you'll probably be pulling the engine anyway, it might be worth investigating such a swap. You can definitely find e300d turbo'd engines of the same vintage which would have all of the goodies you'll need for such a swap.
Food for thought, good luck on your mods
