choosing a position for the boost guage and running the hose through the bulkhead can be tricky, on my 52 plate 45 i could not find the suggested route for the vac hose so just drilled through next to where the clutch resovoir mounts to the bulkhead, if you can be botherd use some grease and a grommet to help prevent rust and potential chafing.
i mounted the boost guage on the top right hand side of the steering column cover using a halfords guage pod, this works as you can remove the base of the mount giving a smaller footprint enabeling you to mount it on the slope or curve of the cover in a position that with the steering wheel straight it is not obscured by the wheel and does not obscure the guages in the dash, you could mount it in the left hand side or add a voltage guage as well.
tip, when using the above pod with the base removed you will have to pre drill holes for the screws cos now you have removed the proper base there arnt any, then once you have chosen the mounting position mark where you want to screw with a braddle spike thing then move the mount out of the way an push the spike through the cover gently so as not to damage any wireing beneth. this makes it easier to mount.