I just did this with my 73. It was nice and no problem whit shipping (it came ups ground).
The install, is not real hard. The boards are made for 73-81, so you will have to cut a peice of card board to fit in the 74-81 seat belt hole. Also the only hole I would cut out before test fitting would be the dome light.
When I got mine all the holes looked to be in the right place I went ahead and cut out all the holes. After getting it in the car and getting the dome light uo I found out my seatbelt holes were in the wrong spot by about 1 to 1-1/2 in forward.
Here is a link to some pics of the back side.