Stowy,
I second Swampy's advice, especially about shrugging or 'turtling' your head. You can think of it as sagging your neck and head down between your shoulder blades, you can think of it as shrugging your shoulders up towards your ears or you can get there through the 'head drop, chin lift' method. While riding on the trainer let your neck go totally slack and let your head fall till you're looking back at your rear cogs. Then without lifting the top of your head, rotate your head back up leading with your chin till you can see far enough ahead (which doesn't have to be very far in your most aggressive race position, just enough to scope the white line and see road obstacles but you don't have to see a quarter mile up the road like we're used to on the road bike, just be careful riding in this position and train in a more conservative head position on busy roads with intersections or driveways).
I also second his observation on power and speed. You have the body type of someone I'd expect to be very 'aerodynamically gifted', long torso not a lot of excess on your frame and based on those photos a flexible lumbar region that lets you get very flat. 300+ watts should take you through 40K in well under an hour. I've broken an hour in the 40K half a dozen times in the past few seasons with anywhere from 260-290 watts and I'm not the most aerodynamic rider with less flexibility and not as lean and long overall. But I have spent a lot of time working on my position and field testing various candidate positions.
If possible reshoot some images taken directly from the side with the camera at least eight feet away and ideally about three feet off the ground and not pointed side to side or angled up or down. So IOW, place the camera on a sturdy table, bookshelf, tripod or some other support and try to set it up so you can recreate the same fitting studio for additional shots as you make changes over time. If you're serious about dialing in your position you should also get an inexpensive spiral notebook and keep a record of key measurements along with things like field testing results, general impressions on comfort and speed and any race or hard training day results. Over time it's what let's you identify patterns and figure out what really works, it also helps you avoid chasing your tail and going back to positions you've already dismissed or getting back to positions that were better if you make some changes that don't work.
Here's an image from some recent TT fitting where I was playing with a longer 'faux superman' style position. You can learn a lot from measuring angles and comparing things like helmet height, torso angles, hip angles, knee angles, etc. Try to shoot something like this and post it here. Having said that, images can be misleading and I've definitely chased some ghosts in terms of ultra low positions that looked incredibly aero but turned out to be pretty slow even though I could ride them and even generate decent power in them. Sometimes looks can be deceiving and even though we can optically measure frontal area with very careful camera setups we can't optically measure Cd and both matter. BTW, videos are better than stills for fitting as we tend to move around when putting real power to the pedals and it allows us to extract frames for things like top and bottom of the pedal stroke or compare deep head tuck with more relaxed positions.
BTW, in terms of field tests if you don't have a power meter than go old school and do roll down tests. Try to find a low traffic street that goes downhill long enough to get you up to or beyond race speeds from a dead stop coasting but then either flattens out for a long distance or ideally goes back uphill. If you find one with a dip followed by a hill then mark your start spot, always start from a standstill and see how far you can coast up the final hill in the aero position before putting a foot down or till you reach a predefined very low speed. If it's flat either see the speed you hit at a certain landmark like a certain telephone pole or streetsign or see how far you coast in your aero position before you drop down to some defined low speed. You can learn a lot by testing positions with this simple test. You still want a calm low to zero wind day and ideally no car traffic during your test runs. Do several runs in each position to reduce testing errors.
-Dave
