Louis Ohland wrote:

I'd like to bore a hole approx 2.75", I have a boring head from APT
which will do it, but cranking the slide over far enough to use either
of the vertical bar mounts leads to significant vibration on my Sieg X3.

Can I use a horizontally mounted boring bar in order to reduce the
rotating offset mass? .500 inch round shanks in a 2.5" head.

The vibration is more likely from tool chatter / rigidity issues than
from the off balance boring head, unless your are running it excessively
fast.