Check the entire fuel line (metal and rubber) for pinholes. In certain locations they won't leak gas when the car is parked, but can allow air to be sucked in with the engine running, causing driveability problems.

You don't have to worry about a vapor recovery system (charcoal canister) These were not introduced until the 1970 models.

I'm not sure what a 68 has, but later 140s have an access plug to clean the screen in the tank. This is separate from the drain plug. The drain plug is small, and usually found near the center of bottom of the tank. The plug for the screen is nearer the outlet fitting, and, if I remember correctly, is about an inch in diameter and has a 3/8" square recess in it (into which the end of a 3/8" drive extension fits perfectly) Run the tank down as far as possible before attempting to unscrew this plug, as all the remaining fuel will pour out the hole when it is removed. It may be very tight, probably not having been touched for 39 years.