After checking the shop manual and finding no information, these are my suggestions. Before replacing the radio in your factory air conditioned Avanti, my suggestion is the check the heater hoses inside the car. They must be soft and supple as you will have to move them to a point of kinking to gain access for the radio. If yours are suspect remove them first and replace them after the radio is changed. If they are in good condition you can proceed with the radio exchange. Remove the right side console panel to gain access to the radio with your hand. Remove the knobs and mounting nuts from the selector shafts and slide the radio back until it clears the dash and drops down on the shelf above the heater. With a light look behind the radio and find the windshield motor control center control shaft. Then turn on the ignition, turn on the wiper switch and watch the arm and turn off the ignition switch when it is pointing straight up. At this point disconnect the battery. You can then remove the radio by turning it so the buttons are pointing to the right and sliding it to the right at about 30* with the center of the side of the radio aligned with the center of the glove box protrusion. This is so the small case screws heads do not hinder the small available space. Once the case clears the glove box corner and the firewall fiberglass lump that accommodates the wiper motor, you can turn the radio 90* from the front of the car. You then push the heater hoses to the right and while keeping pressure on them you can tilt the radio 45* down to you and withdraw the radio, be careful not to let anything touch the push buttons as they are usually quite brittle from age.This is an excellent time to replace the dash speaker also as the radio has to be out to replace it. To replace the radio do as they say in most manuals, "reverse above procedure". It is a good idea to remove the left console cover as well as it makes it much easier to connect the antenna lead and the other power and speaker wires. As a point of reference I replaced the radio in my Avanti with a Studebaker AM-FM unit that measured 7 1/2" deep, 8 1/2" wide and 3 1/2"high. I have heard this unit will not fit but with finesse, patience and no undo force it can be done.