Complex sentence: From May of 1962 until August of 1968 Petty had The Fireballs overdub the Holly material, making them the band he never knew he had though he band had met Holly at Petty's studio in 1958.
Split into simpler sentences: From May of 1962 until August of 1968 Petty had The Fireballs overdub the Holly material, making them the band he never knew he had. The band had met Holly at Petty's studio in 1958.