Other than the seat comfort, one of the most important outfitting things is thigh braces. With the larger cockpit, you can fasten plysood or strip wood to form thigh bracing. Or... like Rob Max does, you can make them out of closedcell foam alone. You want to be able to grip the under side of the deck with your thighs. Unfortunately, on most large cockpits, you thighs will be between the oopening and the deck. Ultimately, you wntto be able to rock the boat right and left hard without you're thighs loosing their grip. It's an absolute necessity for rolling and a necessity for good kayaking technique. Check out commercially made boats - Daggar does a good job.

As far as hip padding (check plates etc) You want them to prevent you from sliding from side to side but you do not want them tight. If they are tight, they will make rolling more difficult and if you want to turn around to look back they will make that even more difficult. They should be there, but you should not feel them while paddling.