I actually have a friend that had to test for a hospital IN a hospital. She was really worried about the security and protocol, things of that nature. So, she diluted. And it worked for her. Mind you -- she's only like 105lbs, 5'6 and what I would call a light smoker.
However, due to your current circumstance, In my opinion its too close to call.
Not only do you smoke a large AMOUNT but from your description but THC is fat soluble and being "stocky" this might work against you. 9 times out of 10, pre-employment screenings are unsupervised so you have a good opportunity to substitute which is far simpler than you think. Look up my posts and see how I did it. But if you're too scared, then try dilution as a hail mary. Regardless, I would try one or the other. Don't use your " real " piss. Unless you wanna risk your job ...