Heir.him does not control the POV. For the Spits (and most other planes) the file Body.msh has a section at the top called [Hooks], in which the 12-element transformation matrix has the X Y and Z values for the virtual viewpoints. these are the last three numbers, where increasing the values moves the POV left, back and up. The values are in meters, and so 0.001 = 1mm.
When I make an adjustment for the first time, I copy and paste the lines of data, placing before the original lines "//" characters or a ";" character so as to render them not readable. This preserves the original values if I bugger things up.
And so if you want your POV farther back by, say, 8cm, you would add 0.08 to the second last number.
As to which line of the two, three or more... The first line is called CAMERA, and this is usually the "sitting farther back" viewpoint. after the is usually CAMERAAIM, which sets the SHF-F1 viewpoint where you're typically centered on and closer to the gun sight.
This stuff is a cakewalk, which any text editor can handle... If you have Body.msh extracted. Note that there's a *possibility* of differences in Body.msh between game versions, IF DT are permitting to introduce changes to meshes in the stock planes. But I'm not at all certain of the practice here, and can only suggest to be aware of a *potential* for unexpected changes if you use an older mesh.