You are right, you really do not want to use a WD hitch that is too heavy (strong) for your tongue weight and the 4" wide A frame you have. Too strong of a WD bar and it can cause issues over time on the frame.
If your loaded tongue weight is 675#, then a 750# or 800# weight distribution bar hitch will work. That it is the WD bar rating. The pull rating may will be a lot more, it needs to be at least 5,500# to cover your GVWR and they will usually end up being rated high enough. But the WD bar rating is what you want to be looking for.
Reese makes a good 800# trunnion bar WD hitch. But there are other good ones out there too. Do you have a brand in mind you like? We can help point you to them.
Also, what type of anti-sway control do you have and is it working very good for you?
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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