Hijack alert:
If you use the later Silca Impero "frame-fit" with the contour end handle:
you only need a nub on the backside of the headtube to mount the pump securely under the TT...and these came in more sizes to fit more of a variety of frames, compared to the earlier Silca Impero with the "classic" handle:
Many folks preferred the classic handle over the frame-fit and most of them bought the size that was "small enough" and used an "umbrella clip" clamped to the seat or top tube, but since the classic handle also has a grommet in the end you could use a peg (clamp-on or brazed-on) instead of an umbrella.
As far as which pump is superior: I own both Silcas (both handle styles) and Zefals (many models) and I like them both (and hate them both) for different reasons.
YRMV