You've probably resolved this by now. But for anyone stumbling onto this thread post 2021, here is some info. I am in the middle of this same conversion on my 1973 Series 3 (single line drums to dual line, vacuum power Assisted discs).

Pangolin4x4 sells a Servo pedal tower that they fabricate. Its about $300 and pretty well made. Comes with the pedal already pressed in, UNF nuts welded in. You still need the springs, pedal rubber and some odds and ends, but it beats trying to find one in a junk yard. Comes bare steel, you need to paint it(or not).

Hope this helps others.