**oops didnt read the 2nd page where you had this all figured out already. **
This might be the solution. I believe it lets you use your old standard thread lines with a metric steering box.
https://leepowersteering.com/product...sae-conversion
I ran into this same issue with the hydroboost unit on my 75 C35. No one sells a hydroboost unit with standard fittings, only the newer oring metric fittings.