Very nice looking ride you have there! Plus one on what Tools said, from what I understand that year CB has the seat hump in the back so you would never be able to put a straight seat on there without doing some frame modifications, I could be wrong but I know that my 81 has that hump there. Have the seat reshaped as suggested, it might be a little thinner in the back for the passenger but it would give you that straight line you are looking for. As always pics if you get it done are a must!
I know enough to get me in trouble.
1981 Yamaha XS650 Special(current cafe, code name El Diablo Rojo)
1981 CM400C, completed project (code name Home Wrecker)
1981 CB750K, completed project (code name Winter Blues)
SOLD 9-21-2013 gone but certainly not forgotten.
2009 USA Choppers Bad Boy, rigid frame custom chopper.