A capacitor can be used to block bass frequencies from reaching the speakers...wired in-line with the speaker + wire...(If my book on how to build your own speaker enclosures was handy I could post the rating you'll need,but its out in my garage at the moment)...
Dont be too hasty to toss the OEM radio if it still works,I have sold quite a few at swap meets to owners wanting to retain "originality"...not for a lot of cash,but 15-25 bucks is better than nothin!...I still have an original GM am/fm/CB in dash stereo,still works but one channel wont play,it probably needs an output transistor...they were pretty popular back in the late 70's and early 80's...I have considered putting it in one of my trucks,the one with no CD player ..