Overfiring?

[J MICHAEL SHAW II <103253.3336@domain.elided>]

I'm a long time IH owner that is a short time web user.  Perhaps you guys
are the answer to my prayers.  I have a '73 Scout II, 345, 2210 carb, AT. 
This one I bought about six months ago ugly as sin, but running fine-except
for the 40-60 accelerator pumps it took to get the thing to start in the
morning.  After about three months I began to have a problem with it
misfiring as I was going down the highway.  The very first time it
happened, I pulled over and looked, thought it might be spark plug wire
cross-fire, and rearrenged a few wires and went back down the road.  I
drove about twenty minutes and it did it again, and has done so ever since.
 It is fine for ten or twenty minutes, then starts coughing and bucking. 
If I pull over it stops, but will start again within about five minutes of
starting up.  It sounds like an extra, mis-timed combustion is taking place
on the passenger side of the engine (just a feel and sound impression),
making the Scout lurch, then decline before catching it's rpm rhythm again.
 Once it starts doing this I can accelerate over it or decelerate under it,
but this leaves me with the option of going 40 or 75, or alternating
between the two in highway traffic.  It gets worse if I'm forced to push it
much farther, until I have to pull it over and sit.  It never dies out, and
doesn't do it at idle, but will eventually start doing it in even the
extreme rpm instances that I mentioned as a way to avoid it.  I first
thought "distributor," but a friend that helps me with mechanical work and
who's opinion I trust over my own said "carbeurator."  Because of the
pumping problem, I thought a new carb was in line, and got a rebuild.  The
problem did not change. I've tuned it up and set the timing and still no
change.  There seems to be no play in the distributor shaft, and my
experience with dist. problems always left me cutting out, not
over-igniting.  I'm now guessing it must be something in the valves
sticking once the engine is warm.  However, there's only 111,000 miles on
it, and my T-all and Father's Scout had many more miles without ever having
any problems.  I'm usually pretty good diagnostically figuring things out
with the peripheral additions to the engine, but have never gone into the
meat of the engine itself.  Any advice?  Valves, or should I try a new
distributor first?  Thanks in advance for any help.
Also, looking for a running engine for my '68 T-all while I restore the
original.  It has a 345, 4-speed set up, though I think anything from those
years would work. I'd prefer a 345, though would consider a 392-I just hate
to cut the little manifold notches in the frame that I think it needs. 
Anyway, this truck is in Taos, NM and I need to get an engine to it so I
can get it up here (San Francisco) and finish it off.  Any suggestions/info
will be greatly appreciated!!!

J. Michael Shaw, II
103253.3336@domain.elided
'73 Scout II-Beater
'68 Travelall-"Old Blue"
'66 Dodge Monaco-Can't get rid of 'til I get the Scout running right!!!