cuda- single to dual conversion-FINE LINES?

... not sure if my description is CLEAR, but what do people do when changing an early A car from single to dual MC?...

What I do is tell them I have a 1970 Duster. The wheelbase is the same and you have a 70 proportioning valve. I tried telling them what I was doing on my 64 Barracuda and all I got was "no guarantee it would fit". Thanks. The lines fit, no help from them.