Chris Hillman was an automatic choice since Tom was a huge Byrds fan and produced Hillman’s solo album Bidin’ My Time in 2017. Herb Pedersen was in the Desert Rose Band with Hillman and toured with Petty and Mudcrutch in 2016. As for The Shelters, Tom Petty got involved with the band very early on, and he ended up co-producing their debut album in 2016. But it was producer/engineer Ryan Ulyate (TPHB’s/Mudcrutch/Chris Hillman/The Shelters) who suggested to Hillman and Pedersen that they do a new version of “Feel A Whole Lot Better” (Hillman played on The Byrds original, and it was the last song recorded for Full Moon Fever) with The Shelters.
In all seriousness, that's about the most gorgeous thing I've ever heard in my life.
[h/t Glenn E. Most]