diftw, Not sure what model 5er you're considering (or if you're keeping the one in your sig), but there are many, many 350 series gas trucks out there pulling some large ones, so I think your payload will be fine with either length bed. Looks like you are comfortable with the characteristics of towing with gas, so I'd say you'll be fine as long as you stick to your guns on the 4:30 gears, which you may have to order for, or did you find a truck on a lot with them? If ordering, you need to hurry as the cut-off date for the 15's is coming up.
I'll add finally that I love my F250, and it tows our TT great, but I do regret not ordering a 350 with 4:30 gears now. That way if I wanted to step up in camper, I could. Good luck.