Hi Scoot,
notice that you are sending both channels from a stereo radio to this single speaker, doubling the input.
Of course it could still be insufficient for good sound, that is the reason why I asked if someone has tried it.
I can give you a different datapoint that might be relevant. I have an 85 wagon with a Becker 754 stereo in it. The 754 puts out 20 watts x 2 channels. The OEM speakers are VERY similar to the kick panel speakers that we use in Pagodas and in 114/115 cars. Someone put 60 watt speakers in my car. It sounded horrible. I put back the little wimpy speakers and it sounds great.
So my test was 20 watts going to a speaker designed for 60 watts and it sounding bad.
The test with the early pinstripe is probalby going to be about 5 watts still going to something that expects 90 watts...
It isn't going to work is my best guess, but I haven't proved it.
If you really want to go down the path of a pinstripe radio with more power, I would explore the option of a Becker with a separate amp, and then using a different amp than the becker amp. I would think that there would have been discussion about this, but I don't remember seeing any....
Scott