From the 1970 Home Preserving & Bottling by Gladys Mann
The purple plums are best for drying; they should be firm, but ripe and quite fresh. They are best dried whole, but if large can be cut in halves and the stones taken out.
Cut plums should be dried with the cut sides uppermost to prevent the juice running out. Spread the plums on trays in a single layer, and dry slowly in a temperature between (this makes me smile) 100-150 degrees F or 40-45 degrees C. If dried in an airing cupboard or oven, leave the door open slightly!!!
If put into too high temperature the skins harden before the pulp has had time to dry out and there is risk of the skins cracking. Dry until no moisture comes out when they are squeezed; this may take a day or two!!
Cool the fruit thoroughly, at room temperature, for several hours before storing. Pack in wooden or cardboard boxes lined with greaseproof paper, pressing the fruit down well, and store in a very dry place.