I am using NAD27 as map datum but the position is about 100ft off
When using NAD27, coordinate conversion tages place between NAD83 (WGS84) and NAD27.
There are two variants, a matrix transformation or tranformation by a datum shift file.
By default Hydromagic is using the matrix transformation. If your map or project is using the datum shift file transformation you have to tell Hydromagic.
This datum shift file is also referred to as NADCON. To change a projection to use NADCON, please refer to this article.