Question

If Roosevelt’s judicial reform had been passed, how would new judges have been appointed?

Answer 1

From what I can research, FDR, back in 1937, had a plan to expand the Supreme Court to as many as 15 judges. He wanted to provide full pay to justices over age 70 who would retire. And if the older justices didn't retire, assistant justices, as long as they had full voting rights, would be appointed to sit with those justices.
So they would've been appointed by age of the current judges.