In response to the question <font color=blue> is this enough weight to keep the tractor stable on hills and with a FEL
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That depends on the hill, as there is a point of no return regardless of the weight, and it depends on what loads you will put in the FEL. Personally, I would go with more weight than 150 on each side for the FEL, or a good ballast box or implement on the 3pt.
Certainly any extra wheel weight will help the stability on the hills. But that doesn't answer the <font color=blue>'enough weight to keep' <font color=black> part of your question, as no one knows what that answer is.