Both of the areas behind the crossmember are where the headers would rub. I wouldn't worry about it now, wait till you decide what you are going to use for a crossmember. You can always box the area if needed. As to the back half of the frame I would look closely to see how it's put together and any bad welding. Might be better off to re-graft a better/cleaner back section onto it. Pretty cheap to buy an old long bed frame. Just a thought.
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