Awesome work on losing 75lbs!
A 2 man ladder should be fine for your weight, if you sit centered, don't go sitting on one side with little weight on the other. That being said you should still wear a harness. At your size a strap beneath your armpits may be more dangerous than nothing at all, you may need to look into something like this :

http://www.probuy.net/products/HUVHARNESS.html

Good luck!