Id love to have that 305 behind my truck!!! I cant speak directly for that model, but towing a 265 is similar, add 4feet in length and a foot in width and about 3000lbs overall. Thats a big load.
My 265 weighed in at 10,600#s on a triple axle 12600# (load capacity) trailer. Thats scaled weight, towing it down the highway. I had a 2004 2500HD Silverado 6.0 gas truck, handled it, but by the #s it was overloaded. My new truck, a 2013 Ford 250 6.7 diesel is a night and day difference. Brakes and suspension are about the same, but power is unbelievable.
How does this compare to your question......I guess it is just some relative close info for end user size and vehicles. If my future boating plans get bigger Id like to have a 305 to tow, it could be done, I certainly wouldn't want to do it with less than a 3/4ton diesel, but I think relatively speaking I would not hesitate (with current truck) towing a 305 if properly set up on a the trailer.
(Im in the process of converting to EoH brakes on my trailer, that plus adding airbags to the truck and I should be ready to roll!!)
Good Luck in your search, I love that 305.