I am trying to install my new astro vents for both the passanger and drivers side. (rectangle vents, not round) on my 69. Does the dash have to be taken apart to take the old vents out and put the new ones in??

You will have to take the dash off to get to the screws holding the back of the vent duct in place.
There are 4 screws with hex heads holding the vent duct to the dash. These must be taken out to get to the vent.
For me the passenger side was the easy side. After 30 min trying to get the drives side out I quit and have not tried to get back in.

Others may chime in with tips.

A 1/4" drive air ratchet with a swivel and an extension can help in the tight areas.

You may also want to cut out some small pieces of felt to use between the vent pivot points. The felt can give the vent a firm setting to hold its position.

You have to take the dash pad and glove box out for sure. you might be able to unscrew and angle the cluster to get to the screws, but it is easier to pull the cluster out.

i just did mine over the winter. yes the dash pad and the dash have to come loose. after pulling the cluster out about two or three inches you can get a 1/4" drive socket behind there and remove the four screws. its a bugger for sure but worth the effort when completed. the pass side is a bit easier as stated after removing the glove box liner. good luck on yopur project.