As has been mentioned, try a fuel filter first. As a kid I drove some of those mechanical pump Cummins powered trucks, and man a slightly plugged fuel filter was awful. I don't recall one that was so bad that it wouldn't rev, but I changed filters plenty on those engines. If you can get it running enough to go down the road, get some Power Service, Howes or whatever diesel injector cleaner you can buy into the tanks.
Those older engines didn't have a whole lot of injection pressure or the tiny holes (in the injectors) that we have today, so not sure how much an injector cleaner will help.