Philip Jamison (MMD 991022) requests a source for large wood screws.
This solution comes from the boat building and marine technology.
(I am a Naval Architect.)

One solution that comes to mind is to use coach screws.  They may be
known as something different in US hardware technology, so here is a
description, based on an Aussie's view of the items.

The coach screw has a single start raised thread, and plain shank,
just like a wood screw.  The difference is that it has a hexagonal head,
instead of the raised head or countersunk head of a wood screw.  Coach
screws are available in a range of diameters (say 1/4" to 1/2", without
looking at any catalogues), and a good range of lengths.  They are
a common hardware shop item, in galvanised or similar plated finish,
on mild steel.  Specialised (very expensive) stainless steel ones are
available from fastenings manufacturers or suppliers to the marine
industry.

What about the hex head?  If it matters, turn the head back (in the
lathe, or with a steady hand and the bench grinder) to round head
(in this application) or countersunk, and cut the screw slot with
a hacksaw.  If not, leave it with hex head.  If you haven't the
facilities, a local machine shop or hobbyist could do it easily.

I haven't tried this alteration myself, but if you do go to slotted
heads, be generous in pre-drilling the holes, as a screwdriver blade
in a straight slot hasn't anything like the torque capacity of a 
spanner on a hex head.

A variant to the coach screw, which probably isn't relevant to Philip's
requirement, is the lag screw.  This is as the coach screw, but instead
of the hex head it has a metal thread, i.e., (from point) wood screw
thread, plain shank, then metal thread.  This allows you to drive the
wood screw thread into the wood, using double nuts, and then secure
(and occasionally remove) the item held down by undoing the metal
thread nuts, leaving the wood screw thread undisturbed.

Best wishes with the solution, whatever it is.

Peter Edmonds
Perth, Western Australia