Have you checked to make sure that your glow plugs work?
My 81 caddy was the same way untill I replaced the glow plugs.
Of coursse there were other issuses. See posts under somolovitch3 in this and othe forums for details.
Cheap fix for runaway, sort of, wire in a normaly closed momentery action push button switch in series with the fuel valve. When the engine starts to runaway, hit the switch, cutting off the fuel.
As mentioned above, be ready to stall the engine if the switch does not help. I ran my 79 Rabitt with a swich set up for several years, would still be doing that, but the poor thing got smushed one day.
If you're getting the engine rebuilt, get the injector pump and injectors rebuilt also. When I got my pump rebuilt, it solved my runaway problem.