1972 ford 390 distributor problem.

I am working on a 1972 ford f250 with a 390 engine that came with no distributor. I bought a 390 distributor ,and it wont fit in the distributor bore. This should be a FE engine .I was told ford made a 391 FT engine that is the exact block , but the distributor housing is a different size. Any help will be appreciated.