Whats in your clutch bellhousing, most likely is oil with which the clutch release bearing was lubricated (it has a small pipe that leads directly above it) contaminated with friction dust that has worn off your clutch plates over the years. You see the same stuff on aluminium car rims, dust coming off the brake pads. But there is allways a little oil slung out of the clutch throwout bearing in those 40 years.
Replacing the gufero seals on the PTO and main shaft wouldnt be a bad idea, as these old gaskets get stiffened in all those years, and when you bend them a bit too far when trying to fiddle the engine and clutch back over the transmission and input shafts, they might deform and leak.
You can take off the figure 8 shaped cover on its own. release the bolts and tap slightly with a hammer to release it from its paper gasket. Then you will hold the 8 shaped thing, with the main input shaft, a bearing and one gearwheel. All plain and simple to take off.
Dont bend that shaft too much when the inner shaft is still inside.