will a gas tank from a 1984 xj6 fit a 1977 xj6?

quite a bit of difference in year modle and body changes i doubt it will .i would try calling a dealer or a shop that works only on forighn cars.

Call your local junkyard or auto parts store that has a Hollander Interchange Manual.  The Hollander manual will tell them if these parts are interchangeable.  Or, you can check www.car-part.com and see what years and models come up on a search there, this will do basically the same thing.