I had a similar problem when my son and I went down to Le Mans last weekend for the Classic. It was only a total of about 250 miles driving each way, but we had a night-stop before a Channel crossing of 5½ hours, as well as a stay of 4 nights in a hotel to prepare for.
The best places to store things are:
Under the front clip, in the odd-shaped space between the nostril and the servos/air horns (some people don't have air horns to make more space - I prefer to be able to warn truck drivers who haven't checked their mirrors properly!).
In the door pockets.
On the sill between the seat and the door.
Under the driver's and passenger's knees.
On the passenger's lap.
In addition to minimalist personal kit, I also took basic tools - spanners, screwdrivers, a set of plugs and spanner, wheel brace, tyre goo aerosol and a 12V air compressor - this last because I have a slight leak from one tyre which needs frequent checking.
One invaluable piece of kit, if you can find it, comprises a heavy plastic bag with an airtight seal at one end. The other end has a number of one-way valves. Put your kit in, seal the bag and roll it up tight. The air goes out through the valves and - instant vacuum-packing! Sweaters and soft clothes shrink amazingly. The main advantage though, is that you end up with a flat, low bag that fits snugly and very comfortably under the passenger's knees. Mine is called an 'Eagle Creek Pack-it Compressor Set'. I put all my stuff into just one of these, whereas my son had a couple of small rucksacks, which were a bit awkward because their round shape doesn't lend themselves to fitting into GT40 storage areas, which are mostly rectangular or flat.
Still, it was an interesting experience; my son said he was so comfortable in his 'coccoon' that a couple of times he chose to stay in the car when we stopped. Whether it was because of the aggro in unpacking/repacking or because he really was comfortable is debatable!
They say that a GT40 turns a journey into an adventure - the Le Mans trip was certainly that!
