Twice in my life I have seen where too long of a bolt was used in the last hole back on the left side of the intake manifold. The bolt hole is drilled all the way through, when too long a bolt is used it goes all the way through and hits the push rod on #8 cylinder.

Thanks for all the advice.  It was tricky actually measuring valve lift.  In the end I ran a long straight rule across the rockers above the springs.  They all seemed to move equivalent amounts, but I'd need to confirm that.  It doesn't run rough, although it could be a little smoother.  It accelerates well, but driving it around it sounds like there's a miss.  I'm going to accurately measure all compressions to be absolutely sure.  I cranked the engine for about 20 seconds.  No wetness on any of the plugs on the right bank - a little on the left bank.  I'm wondering whether there's a vacuum leak around the inlet manifold to the head near cylinder #8.  Would that cause this problem?

Yes, a gasket leak could cause it.  Try spraying some volatile liquid, like choke cleaner around the area with the engine running.  If the engine reacts, there's a leak.  Do this outside, you'll be spraying near the distributor where there are sparks.  I usually use an unlighted propane torch to find a vacuum leak, but have had an occasional flash of flame if I allow the gas to accumulate.  No harm done, but my heart rate jumped a little.

John - "The Hoosier Hurricane"

Thanks guys.  A few things.  As Sandbird mentions, there is a longer bolt than acceptable in the last back hole of the intake manifold.  Doesn't hit the pushrod, but is a little close.  John - I'll try spraying some volatile liquid around the manifold near cylinder #8.  I re-ran compression tests on all cylinders - all got within 135-140psi, so pretty good.  As Charlie suggested - I've checked tappet clearances, screw adjuster positions, etc. - all good too and valve lift seemed fine.   I also used my leak down tester to check that both inlet and exhaust valves open and close, which they do.

It really has to come down to a vacuum leak near that cylinder.  I've ordered a gasket set from the east coast (of Australia).  Hopefully I'll get that later this week.  I guess the inlet manifold will get a new coat of paint in the meantime.

I recently received intake manifold gaskets and fitted those.  I cleaned up the faces of the heads and intake manifolds until they were sparkling.  However, the problem was still there when I started her back up - no drop off in revs at #8 cylinder when I disconnect the lead.  Spark and lead both good.  I then started spraying around with some "Start Ya Bastard" (a highly volatile engine starter in a can).  Nothing around the manifold to head connection.  I then noticed a change in revs when I sprayed the mushroom-like fitting on the driver's side.  I believe that's a cap to the hot air tube that runs across the manifold to the choke assembly.  Is it possible that the pipe could corrode inside the manifold?  But then why would it affect the intake and not the exhaust?  It's hard to tell exactly where it runs through.  I removed the mushroom cap, cleaned it solvent and sealed it with solder.  When I refitted the mushroom cap, there did seem to be a noticeable improvement.  That is, the revs dropped when I disconnected the lead to cylinder #8.  Does it run through the inlet plenum as well?  Maybe I'll cap the side to the auto choke and fit a manual choke, based on advice received.

If You want to keep the Auto Choke there is an aftermarket which simply runs a hot inlet pipe to the Exhaust Manifold like a lot of 1960's Fords did. This was a Kit sold to do away with the choke Pipes through the Intake setup. Probably find One on Ebay.