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I'm not sure when ford changed it, mabe 84? but the sending unit/pump hole went from small to large hole. 91 is efi with 1 pump, not sure what engine your 86 has, I6 has mechanical fuel pump on engine all others have electric fuel pump. the fuel pump system changed in 90,old system had lift pump in tank, high pressure pump on frame. 90 and newer had pump in tank only. back tank will physically fit for sure, front tank may be too long.