Try Instrument Specialties. Sorry I don't have their contact information. Costs about $650 and 10-12 weeks turnaround time. Featured in the mopar muscle magazine last winter I believe.
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I'm running the 275/60R15 ET Streets on an 8"rim (3.75" backspacing) in the back and 205/55R15 in the front on my '73. The rears were on a 7" rim previously.