You always change the gaskets and O rings, so yes you do use and need new ones on the cylinder bases. I always change cam bearings anytime I change cams, cheap insurance. Roller rockers, big question. Roller rockers will help correct for screwed up valve train geometry, will decrease valve stem tip wear, reduce frictional losses, give you an exspensive item to discuss while bench racing, etc. Do they work? Yep. Are they necessary, no. With your setup I wouldn't bother, unless you just have the money and want them. As for claying the setup, I honestly have to admit to never installing a set of 251's yet. People seem to go much larger with me, or they just use the 203's etc. The HTTC heads and pistons did have some clearance issues when they came out. However I believe that is not an issue anymore, and I believe there is not an issue with the 251's in your setup. However I would seek a different opinion for clarification as I bailed on the whole harley thing about 5 months ago and cant remember half of it. Over all I think your set up is good, the only thing I would consider is a different exhaust, however your bike will run great anyway.