Steve,

Don't centre the bar on the 150" post. Start the reinforcing bar on
the base plate and run it half way up the post. Weld securely to the
base plate also.

Personally I'd use two bars and weld them to the back corners, at 45
degrees to the back side of the column.

From a column stability perspective 150" un-reinforced is too long.
Maximum unsupported length is 8.33 ft as per my CISC handbook.

Wolfgang