255 is a nice cam for a bagger; think I would opt for another cam for an FX model. The 255, with a good tune and good exhaust is capable of developing 110-115TQ but it will peak around 3800and HP won't be much more than about 85. If you are stuck on SE cams, take a look at the 251; won't come on as early on the bottom end, exhaust sensitive so you need a good 2:1 system. Will make power to the rev limiter.
If not stuck to SE cams, take a look at the Andrews 54H.
It is important to select the cam before you start building and build around the cam. The heads flow fairly well and don't need much work but decking a few thousands off for the compression that makes the cam happy could be required. Flat top pistons and heads decked for compresion, IMHO, make for a more docile, well mannered and easier to tune engine.
Do the homework; ask questions here or other forums and get the build togehter in your head before you pull the trigger; the results will be predictable, it won't cost as much and, hopefully, no "do overs".
Just occurred to me that with an FX model you could consider the SE204. It gets a little "smallish" in a 103 and your tech will probably scoff at the idea but the 204 is a very underrated cam, IMHO. I ran one in a 95" Deuce one season and it made 104TQ/94HP and surprised several bigger builds. Nice choppy idle and, in spite of what some will tell you, if set up and TUNED right, it will pull to the rev limiter. Not a good choice for a bagger but for an FX, it could work.