450 prefix can also be '74 DT250 (and maybe twin shock '76?). The actual serial number tells you which year, but I can't remember what the change-over number is.
Other way to tell is the cylinder head - '74 has parallel fins, '75 has radial fins.
Speed and Sport shows no matches for 3J5 or 3O5.
305 prefix is LT2, which you wouldn't mistake for a 400cc engine!
Most barrels will have the capacity cast into them, which would help.
Pics?