I think you're on the right track with 800 lbs bars.
As far as under / over size?
I've always heard, you don't want to under size at all.
Oversize amount is questionable. Some manufacturers have a very large jump between sizes. Some have bars rated in one hundred lb increments, some have them in two hundred lb increments, and some have larger jumps.
Blue Ox for example, has bars rated for 600, 800, 1000, 1500, and 2000 lbs. If your tongue weight is just over 1000 lbs, you would need to jump to 1500 lb bars.
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