Check out the forum on the website below, some useful tips and location guides
http://www.flylow-flyfast.co.uk/
As you mentioned, weather will play a big part in your day, 8 0'clock abit early as the weather hawk doesn't usually go through the pass until about 8:15, but its up to you. The Bwlch has 4 locations, the shelves, personally the best position is the top shelf, as you get to look down on the aircraft, there are the middle and lower shelves each offering a slightly different perspective and the Bwlch Exit. If the conditions are perfect, clear skies and sun, then your lens will cope fine, but if the conditions are overcast, then unfortunately the bigma will struggle, f6.3 just won't cut the mustard when the conditions in the loop aren't in your favour because you'll need to keep shutter speeds reasonable high, 1/800 1/1000 or 1/1250 for the jets. As for a monopod, I personally wouldn't recommend one, unlike motor racing where you can guarantee the subject is in the same position every lap, you can't with lowfly, also it could be quite easy to lose you footing if you get tangled up with the monopod, and humans don't have feathers and wings. I handhold the canon 300mm f2.8 (@ 2.5kg) a friend the 400mm f2.8 (@ 5.6kg) for lowfly with no problems.
This isn't like an airshow, aircraft ain't appearing every 15 minutes through out the day only stopping for pie and chips at lunch time, its the waiting game, which could all kick off 6/7 hours after you arrive, but then again, there could be a steady stream of aircraft, it the unpredictability of what's going to happen next as well as the buzz of capturing those images that drags us back everytime, F15, inverted Jaguar, Tonka coming out of the mist and the afterburners

enjoy your day on the loop