here in Pac NW, downriggers for salmon are a must.
Almost every boat i've owned, i've mounted downriggers (DRs) and every unique boat has its DRs mounted in it's own unique spot on the gunwale.
where you mount them is usually dictated by the kind of fishing you plan to do, where there's decent access to mount them, and in a location that is friendly to using the DRs.
on my 282, i've chosen to mount them about 6" in front of the rear rod holders, but this limits the boat's tight turning ability as even with 60" downrigger booms, i must be careful to not place my gear into the props--so, i just don't do tight tight turns.
you can mount the DRs either further forward (i've seen on some boats the DR's mounted almost amidship) or as far aft as possible to further reduce the issue of line and gear running into the props. If you choose to go closer to the forward gunwale rodholders then you may have the line rub on the aft chime of the boat during turns so you need to be running spectra/dyneema DR line and not SS cable as the SS would damage the boat.
i've posted a few pics of my 282 with DR's for your use. after a couple season, i'm very happy with the location i've chosen.
good luck.