Replace the filter, check to see if there is any gasoline in the oil. If
there is, the fuel pump is leaking gasoline into the crankcase. The next
likely suspect place is the carburator.

Of course through the newsgroup it's hard to know what the 'sputter' is
like, so an ignition issue might be in play as well.

You didn't say, but it might be helpful, to know how well the car functions
under the 45-50MPH range. For example, how does it start and idle and  how
does it accelerate before you reach the point it starts to sputter?

For what it's worth, a few years ago I bought a 67 Mustang coupe with a six,
that had also sat for a number of years, so I can relate to what your going
through. My car had similar problems and eventually I discovered that the
cause was no vacuum advance. The car would start and idle ok but at some
point while accelerating it would, like yours, start to bog down, sputter
and die. Replaced the vacuum advance diaphragm thing that attaches to the
distributor and the problem went away. Incidentally, I did add a see through
inline gas line filter and it was amazed by the amount of rust that was
coming from the gas tank. I did replace the little cartridge in the filter a
number of times but eventually the gas cleared up so I did not replace the
tank and It has been working fine since.

Gene from Georgia

   It's probably the fuel pump.  They are cheap and easy to replace.  I'de start there.

Don't forget to check for crap in your float bowls! I had a Ranchero with a
carter carb that would get clogged with junk that would sneak past the
filter.
Good luck,
Dave

   Could just be bad gas.  Try adding a small bottle of isopropyl alcohol.

   Also, similar symptoms to a bad fuel pump.  After a while, the rubber diaphragm
in the pump dries up and volume and pressure drop as the rpm's climb.