I can't help you on the first one but 6'' is pretty common for a stack on a 250, if you use 8'' it will be really short. I've saw quite a few 250's with single doors. I'm not sure of the weight but you can add a counterweight and it'll open with ease.
To find the center of the head,I would first find the 0, 90, 180, and 270 degree marks on the cylinder of the tank and draw a line the length of the tank at each point. You should then be able to line up a laser level over the end of the dome with the 0 and 90 degree points that connects them. Do this with the 90 and 270 degree as well and the intersection should be your center point.
My laser level sits on on tripod and has a vertical and horizontal beam which makes this a pretty quick solution but you could run a string around the ends with fairly good results as well.