The 100-400 is a little slow, so if the weather sucks or you want to isolate your subject from the background it's not the best choice. Optically it's good, but nowhere near a prime. The main advantage is that it's small and light enough to shoot hand-held for an entire race.
Yes, the 1-4 could be considered a tad slow, due to its Super-Zoom/ Variable AF feature, but on a Bright & Sunny or Bright & Overcast' Day, it does fairly well. Plus, the OP, does have the option of increasing the ISO, to keep the S/S up, if he so desires. But as you also pointed out, its form-factor, keeps it from getting too heavy over the course of a long day.
I'd still go with a 300/2.8 though, just for the subject isolation and the chance to get pix if it's cloudy or if things go late, as well as the fact that the camera body will get to focus at f/2.8, which can make or break quick focus on moving vehicles versus f/4.5.
The 300 does a great job w/ isolation. I agree 1000%, though I might be a bit biased.
However, though the 3 is lighter/ more agile than its larger siblings (4, 5, 6, & 8), they all take some getting used to, and I don't believe a first time rental before a race, would be the best time to learn the curve, with the Big Whites.
If you're one of those people who thinks a 24-70/ Da' BRICK is heavy then the 100-400 may be a better choice for you.
BINGO. I'm not here to judge/ make assumptions, but we don't know what the OP has or hasn't used, thus we can only make recommendations, based on our own experiences. Hence the reason, I was recommending the 1-4, as IMHO, its a step above the 7-2/4 that was suggested.
OP, by all means if you've used the lenses that give you a workout, when using them, then go for the Big Whites.
However, if you haven't then, I'll stick to my recommendation of the 100-400.
OP, also just an FYI, one of the reasons' I suggested the grip, is the Rebel doesn't really provide very good balance on long lenses. IMO, here are two important features a grip does for you, (1) it permits you to shoot vertical w/o making your wrist sore (been there, done that), and (2) it provides more balance when mounted on the longer telephoto lenses.
Just to be contrary, I've had no problems at all trackside with my 400 on a Gitzo 1548 + Wimberly head.
.
Just to Clarify;
Are you using a 400/2.8 IS, on a GT1548 & Wimberley?
OR
Are you using a 400/5.6, on a GT1548 & Wimberley?