Ouch! Well then Mike'* right. you need to easy does it and tranny cooler necessary. Why a bed and dresser 600 miles?

One of my kids moving to a new job and wants to take her bedroom furniture.....sentimental thingy...

Ya, I had the scenario all wrong. Stretching the towing capicity for 600 highway miles isn't a good idea.

A shift kit would really help. Nice firm shifting instead of the drawn out shifts would be a great improvement, not just for hauling but for daily driveability and longevity. Just remember, 1st,2nd,D, and OD are there for a reason. Dont just drive the whole way in OD. use the gears as they were made to be used. My last bonneville, a 1993 SE Custom had a shift kit, and i drove it like a manual tranny, and beat the crud out of that car, and i never had a problem with the tranny at all

I don't know if all this work is necessary for 1 trip. If it is just one trip as I get the impression of, the only thing I would be inclined to do is install a transmission cooler. You could probably get away with not running one on one trip, provided you drive smart. Also keep in mind that the brakes are not going to have much fun either, so it would be wise to take it easy in all aspects. Does it have to be a 5x8? Those units are fairly tall. I would feel better with the 4x8 in all honesty.