I personally attempted the 100 nugget challenge and the bottleneck for me actually wasn't getting full, it was that your jaw hurts a lot chewing through a shit ton of nuggets. I couldnt even imagine doing over 4x that amount in a sitting
Just sounds like you need to start training by chewing gum at least 8 hours a day. And none of that Dentyne stuff, you need to find the cheapest pizza shop with the "Pink Gumball = Free Pizza!" gumball machines and chew those. Give it 6 months and 400 nuggets will seem like nothing.