I've used this type of adapter in the center speaker opening
Halfprice audio
Halfprice audio
I've used this type of adapter in the center speaker opening
Halfprice audio
Some advice, don't mix cheap speakers with better ones, especially if the impedance between the two sets doesn't match. The cheap speakers will limit the performance of the good speakers because the cheap ones will distort much sooner. You are probably better off not installing the dash speakers at all instead of doing that. Quality sound is 90% speaker quality. Don't spend all this time and then be really unhappy.
You might try pulling a door panel and seeing if the door frame will take a 4", 6" round or even a 6" x 9". You would have to either buy aftermarket door panels or you could cut a hole in the panels you have now and mount a grill over the hole. I cut the door panels for a 65 GTO I had and it worked a charm.
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