300mm lens suggestions Circle track racing production

Just bought the Pocket 6K and will be shooting 135MPH race cars at night.The lights at some tracks are a crap shoot. I have 2 days to get a lens and do not know the lighting at this facility.Im just going to cut to the chase here. Need some guidance on a 300mm lens for this Camera and... will i find that I need a booster for shooting 4-6k in this environment? Thanks. Im a Novice with little time to make a smart buy.

A Canon 300mm f/2.8 should do the trick, itis a pro sports lens, their is a used on at KEH https://www.keh.com/shop/canon-4411b002 ... gKXbPD_BwE

Thanks Denny!
I ended up with a 70-300L f4 for the race in Idaho and was floored at the results. Am currently ecstatic at what the 300 will produce. Learning more daily about this BMPCC and loving it. Just need deeper pockets now!

or try the Sigma 120-300 f2.8 for more versatilit?