If you had a restrictive exhaust, the engine would knida buck and stumble, due to the back pressure. Remember the kids putting the potato in the tailpipe? even a partial obstruction would hinder engine performance, like a miss in the cylinder or a back fire through the carb. running rough things like that. You stated in 3rd the engine runs fine and just won't go past 40mph or so.
The gearshifting on these old cars goes about like this, 1st 0-15 mph, 2nd 15-25/30, 3rd 30 on up. In 3rd the torque output from the engine is over riding the grip of the clutch plate to the flywheel. I just changed my clutch assy 3-4 months ago for that very problem. Now it will climb a tree in 3rd.
Joe Lee