With a starting point of 1945, how would it be possibly for a country in Sub Suharan Africa to become a great power, comparable to current Britain or France?
Obviously, they would need a decent population. Nigeria has a huge population, while the Democratic Republic of the Congo and South Africa have large populations as well. They would also need an economic boom-which would definately be possible with 65 years to work with under the right conditions.
Was it possible?
Unless either the British or French enable a smooth transition to a stable democracy, it is unlikely to be a viable option, as the underlying causes of most African problems were tribalism, big man politics, and a lack of care in the colonial architectural legacy.
That said, if you want a country which could rise Phoenix like from the post-colonial world Kenya, South Africa, and Congo would all be good candidates. All three have the infrastructure in place to take advantage of the resources on their doorsteps in the 1950's and 1960's, and if the political situation were to change (such as Patrice Lamumba being allowed to continue his rule), then all three have a chance at economic development akin to South Korea or Brazil. Don't forget that SA had a nuclear arsenal till the 1980's, which it gave up.
The biggest thorn in the idea of a post colonial Sub-Saharan power bloc is that is does not suit the rulers of the countries involved. South Africa in particular did not like the idea of Black Africans getting ahead in the world, and interfered in the internal politics of its neighbours on more than one occasion. Rhodesia could have made a go of it, but the post colonial anti-white movement shawn the country of the intellectual currency it would have needed to thrive. Kenya squandered its infrastructural legacy by not spending on repairs to roads, railways, and other key areas of the economy.