Ian,
Forget about those little square factory speakers in the footwells. My '72 250 had those with the original Becker Europa stereo radio. Even when new, they sound horrible. My solution was to mount a pair of 6x9" 2-way speakers in the cardboard covers under the dash - above each footwell area. If you don't have the cardboard anymore or don't want to cut holes in your original ones, one of the front speakers could go in the factory speaker hole on top of the dash (you would need the factory size speaker or make some kind of adapter to fit a different size there).
You could hang a small cube speaker, up in the front passenger footwell for the other stereo channel. Hope this helps.

Happy Motoring, Mark

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DrDKW