the hang on A/C works really good as the evaporator is huge in them and can make the car an ice cream freezer if you want (dont try that though).
To make the original duct system work, you need to find the lever and button assembly from a 1980 corolla (1982 also fit but is different in looks) - you will also need to find the entire HVAC box, blower and evaporator coil box for this car and the centre vents for the dashboard too,
The body has provisions for all the bolts and holes for the pipes (the A/C pipe holes are missing and need to be drilled out)
Removing the dashboard in this car is very easy, dont worry on that. The major problem is sourcing the parts because this car is very old now and a lot of them were sold without A/C when new, the A/C models were called GL and SE and are very hard to find now.
You can try to find a whole 1982 corolla dashboard and HVAC box, if fits bolt to bolt on this car.
The sanden 505 fits perfectly on this engine, fit the largest PF condenser you can find and install an electric fan on it, - replace the engine fan with a clutch fan and replace the shroud for it too. If the car is original till now, you will find the A/C compressor wiring in the fuse box on the battery mount, Its end point is near the ignition coil. Most cars will have it cut off because the wiring was exposed to water from missing undershields in the fenders.
Its a very easy project to execute once you have the parts