Although this doesn't answer your question on if lock washers are required or not, I did come across this possibly helpful info about the torque specs for the manifolds.
According to this list
] (see Note 1 below chart) the torque values are indicated as:
End clamps 20ft.-lbs., center bolts 30ft.lbs.
Also found this previous Stovebolt conversation on 250 torque specs
. There is an old but still active link in there to GM I6 engine torque specs for 1966 that should also apply to your '67 250. See the screenshot below for details.
I don't have a Chevy shop manual covering the '67 250 engines, so can't confirm if these are accurate or not???
Anyone else have that resource to confirm???