I understand the foundation for a developed middle class was non existent in 1960 but surely the MASSIVE mineral deposits that exist throughout the country could spur a extremely lucrative "cash crop" economy to rival any nation on the continent.
That would help some, in so much as it could provide revenue to support a government that can provide stability. As others have mentioned though, resource extraction is prone to booms and busts which mean cash crop/mineral extraction based economies tend to have great difficulty getting out of the trap of being an exploitation site. Something else is needed to give an extra push.
IMO more urbanization with various service enterprises and small scale industries growing up to serve the city itself, thus supporting a small and secure middle class, might be the first step on the road to real endogenous growth.
A very good timeline focused on the Congo becoming a stable and successful nation is Onkel Willie's Freedom and Brotherhood which you should check out.
I hadn't run into that one before.
fasquardon