You need a sending unit for a 77 Aspen?
Is it the later style or does the car have a round spare tire well?
I think the only difference between the late 77F and 79 or so and newer is its a 2 wire plug. The ground isnt a separate wire.
I prob have a good one for the late 77 up F body.
If its the 76 thru mid 77 style your likely S.O.L. Not only is the tank and sending unit different youd need to change the trunk pan to use the later tank properly.