I just did this to a Varsity last week using Deore Derailleurs. Be sure you've got everything screwed down well....I broke the mounting bolt of a cable stop doing exactly this last week.

My solution was to disassemble the original FD and remove the cage. Then I used the remaining clamp and Cable stop that was part of the original FD as a cable stop mounted directly below the new FD.

I'm on my way home in a couple of minutes, I'll take some pics to illustrate...but its very similar to Pastorbob's solution.... I actually tried the problem solvers clamps but found the mounting screw to be very weak and accidentally broke it.