3 point belts on 65 Dart Convertible

Tracking. This is very timely for me as I was just going to ask the same question. We have a '64 HT Dart GT and need to put good belts in it for our teenager. Is it true you should not put in shoulder belts if you have theNON-locking seat back type buckets??
Like these, but in a hard top.
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