If all you're gonna do is longline troll, then a few more rod holders should do the trick. But, if you want to be able to Push, Pull cranks or jigs ... or Spider Rig out of it, then a set of 4rod rod holders at each of the four corners of the boat would be my suggestion. You could even longline troll off of them, too. Varying the length of rods used, or spreading the rod positions will help keep the lines/baits from running too close to each other.
Everybody has their favorite, when it comes to rod holder brands ... so I'm not gonna recommend any one over the other. I'm currently using Hi-Tek rod holders, myself, and they do a fine job.
Here's the only pic I have that shows them in action :
Attachment 275342
(my boat is a 1999 16' ProCraft w/90Merc 2strk, and will run about 50mph WOT ... so you may get more speed out of yours than you expect :dono)

