stock banshee wet = 412lbs (owners manual)

I am not sure, you might want to find someone that is running a set up like you are describing and find out what they learned building theres. what the week parts are that tend to fail.

initial thoughts, you start stretching parts, you increase the mechanical advantage, so if stock, a force of 1, and you stretch it out, what is the resultant force? obviously greater than 1, but how much?

components tend to fail at the worst possible time...

Realizing you are on a budget, you nailed it, safety is first. it might take longer to do it right, but it will be well worth the time and money.

What you just described is a full out atv. Me, if I needed to extend the frame, I would build a new one, run the math on it to make sure it will handle the stress with the desired dimentions...

At my age, I would build something with a roll cage