I would think that the 84 would have more inlets and outlets due to the return line and extra fuel pump of the EFI. That being said, alot of advertisements I have seen offer the same tank for all years of broncos. Call ford and ask the parts numbers of the 2 years.
What I should have asked is if anyone had tried this. Fuel injection wasn't an issue until 87 (I think). The Ford part #'s for the tanks and sending units are both different, but I thought it still might work.