I forgot to mention, since you're interested in the seat and tail light.......
You have to weld on brackets on the frame to add both. For the seat, I just laid a piece of flat stock steel where the seat posts sit. The spacing between the seat posts are 7", so you need basically need to add 1" to each side of the frame. Running the flat stock all the way across the frame will give it the best structure anyways. For the tail light, I cut off the rear supports from the frame and welded the flat stock butted against those cuts, then rounded it off real nice. I'm not the BEST or CLEANEST welder, but after I made sure it was welded good, I smoothed everything out with some bondo.. The bolts on the tail light are spaced too far apart to fit on the bracket, so you have to rig a bracket for the light to mount to, then mount that bracket to the frame.
I'll take some pictures of all of that once I get my bike back. I took my bike to a reputable dealership, but they apparently hire shade tree mechanics! Took a week to order parts, then while putting the head back together, they ripped the head gasket... That was a week ago. Still waiting to get the beast back. I'm especially pissed since it's a darn near 60 degree Saturday!