Bike weight limits

Hello!
I'm currently looking at purchasing a new bike, as my Kent 29 Walmart bike (that I got for $25 brand new :x) probably won't last as long as I'll need it.
I'm currently 345lbs (6ft), down from 373 since febuary, and am falling in love with riding.
I've had my eye on a few bikes, but I'm not sure if it can take my weight. So I'm coming here for help. Even though there are thousands of post identical to this one. I've read that a lot of bikes are built to surpass their recommended weight, so I've been looking at a couple of options.
The Trek Marlin series is on top of my list. Marlin 5, or 7 in particular. It states on the website that the maximum weight of cargo, bike, and passenger is 300lb, which is discouraging. Are there any modifications I could do to it out of the box, to be able to hold up my weight? Or would the frame not be strong enough to hold me. The bike I have now is doing just fine at holding me, but it's got dual suspension thats a bit squishy, and I'm kind of afraid to push it to more than a casual back yard trail.
The Raleigh Tokul series as well, I can't find anything that's telling me a max weight, but I'm seeing good reviews online. Though I'm more comfortable with the 29 size, as I'm a big guy.

Thoughts? Suggestions? I live in Atlanta and there are a good amount of trails that I'm dying to try out, but I'm not confident in my bike.
Thanks in advance!