Yea that appears to be more of a later 60's frame. fender mounts, sprung headtube etc... That number does put it in the middle of recorded numbers from lil indian for 1967, That's not always been 100% accurate way to id, but I have found that to hold pretty solid and close.
Almost looks like grinding was done on the right side, of the frame so it may have been a 600 in another life. but its pretty hard to tell as it has some work done to to it all over the place.
They used Briggs at that point only, The offered a 2 1/4hp on the 400 and a 3hp on the 450-500-600 bikes in both 67 and 68, If its a '67 it would have usually used a larger brake caliper, with the standard wider bolt spacing (K-H/H-H 200) of course those are just about as hard to find as the 1968-72 style with the smaller spacing anymore in correct configuration. The bracket is a little taller than the norm with a half moon cutout in it. They appeared to transition into the smaller 1" bolt spacing brake going into 1968 IIRC