The speakers are easy to remove. The lower dash speaker grills are held in place by two small cross head screws at bottom and two metal tangs that keep the top in place as they slide under a metal arm.
Remove the screws and slide it down.

installation is greatly simplified by using a plastic card ( store loyalty or similar ). Slide it from the top under the metal bar , slide the grill tangs so the card sits between them and the grill body and gently pull the card out while sliding the grill up. Then screw into place.

Many modern speakers have a large magnet requiring some enlargement of the speaker cavity - especially the driver side one.

I used a pair of alpine speakers - sxe 1025s. They sound great.

To keep the period look I installed a refurbished blaupunkt cassette radio that has been modified to accept an aux connection.

Very pleased with the end result.