the chassiz/brakes will handle that much of a load behind it without breaking a sweat. I've pulled upwards of 25000 lbs behind an F250. (diesel)
The 351 will be slow and/or hard on gas pulling 8k but it'll do it. fwiw.

A lot of the 351 trucks especially later on with the E4OD have 4:10 gear ratio. I even have one in my yard thats a 4:10 with a C6 trans. I could only imagine how bad on gas driving that one on the road towing something would be.