First, instead of only motor size, you want to consider motor size, transmission (manual/automatic/3 spd/4 spd/with or without auxilliary cooler), and differential (rear end) ratio. You told us about trailer weight and hills, but how often are you going to pull that trailer, how far will you be going, and at what speeds (highway, rural roads, city streets).
But without any other information, I'd think a 454 with a 4.10 rear end would be good, but the 350 can certainly do the job in many circumstances.
It is an automatic and a 3.73 rear. Trailering at least several times a week with an average round tripe of about 30 miles. Like I said, lots of hills, some steep. About 50% highway, 50% rural dirt / rock roads.