the turbo box is differnt in it can mount the heat shield but you dont really need that so you could use any regal/monte/olds /pontiac elcamino setup and the boxes are the same on those and bolt right up to the firewall .
the harness is different in that the vac lines change over to black plastic at the grommet where they enter the engine bay but work the same
the earlier ac boxes had differing configurations on the ac evaporator core so if you find an older than 86-87 box youll need to change out the ac evaporator core
if goal is to restore AC. not a bad idea to replace the heater core while you're at it
as for install you just need to clear the center support bar behind the dash(behind radio on right side) ,
remove all the screws from the dash top plate and then from dash top into body , remove the bolts at bottom corners of dash , drop the steering column , unbolt the center bar and pull the dash away on passenger side (dont need to do a full dash disconnect and removal ) ,
if you are missing the ducts in the dash for the defrost or the driverside behind gauges youll need to get those back in,
slide the replacement box in and bolt it down
put the dash back together
plug in the hvac controller under dash and the connector at back of passenger fender liner and you should have blower
hook up the heater lines and vac lines and you will have heat /defrost and vents
i had a delete plate and wanted some air /heat and defroster , i went with a heater only box but for the work involved finding one and the amount of difference in room under hood with ac vs heater only if i had to do again i would just do the ac box