Yes, the joy of a small welder! Short duty cycles. Although, that can be a blessing in disguise, by forcing you to go slower and not get things to hot. It's really easy to get excited on projects like that and just weld things up as fast as you can, but that can be bad. All the heat can cause warping. And if your structure is angular enough to not warp, then you can be putting a tremendous amount of stored stress into the metal. The stress is invisible, and it can look great, but eventually it can crack and fail. Especially on something like a bumper that will get winched and pulled on.
Great idea on gusseting the mounts. I'm actually surprised they didn't design it that way. That will make a huge difference in the strength of the mounts.
And like Woodtick said, we're just trying to help you do the best you can! Hope you don't think we're dogging on you! It's always cool to do your own stuff!