your gonna tow, so I'd think torque and service,
I'd build a 489 as it's only a 30 overbore.. the 496 is 60 over and with towing heat , you don't want the cyl walls thinner that you have to..
oval heads , depending on gearing.. dare I say a peanut port top end. with a hyd roller for low to mid range torque.. forget hp.. you'll mostlikely never see north of 3500rpm..
if building with p port heads scares you. I'd use g.m. oval port heads, and shoot for 8.8 to 1.
remember you'll be towing, and building tons of heat..
I'd even think of adding a water/meth injection for every punch over hills..