My experience is that the dash needs to come out. No big deal with the Alfetta GT. 4 screws hold it in. 2 thru the defroster vents, 2 on each side footwell. I can't remember what electric connections there are beside the tach and the instrument cluster connections. I have added several electric things like a starter button, voltmeter, and something else.
Removal is a little easier with the steering wheel removed, as well as a 2nd set of hands to steady it as you slide it up and out. But 1 person with steady hands can do it. Installation really needs a second person.
The HVAC/heater box is bolted to the firewall and access is blocked by the dash. You may find that some of the box may have broken plastic tabs so that it is actually held in by 3 instead of 4 screws. I just made a metal flange/long skinny washer to make up for it.
In case you need AC connections replaced or whatever, having the dash out will make your HVAC shop happier if you deliver the car without the dash, just the heater/HVAC box installed. I just hung the instrument cluster in place for this.
Paul Blankenship AROSC
76 Alfetta GT -Track and daily driver; 88 Milano - a continuing project