318 Bogging at 55mph

What happens if you drive it in first gear ?
It should do the same thing driving it in first gear just a block or two.
if it is a fuel problem.
Make sure your headers are not getting your fuel line hot and vapor locking.

A plugged up muffler will keep it down like a bad cat converter.
I am sure a tec will chime in here and get you going.
Just my 2 cents worth. A friend of mine had the same problem on a chebby and his cat converter was glowing red and he was wondering why he could not get his car over 50 mph.:-D