more cubic inches is always better, pretty much no matter the intent
personally I would build the 489 or 496 since you can buy a 4.250" cast steel crank instead of having to fork out the money for a forged crank.
a decent "claimer" style rotating assy with a cast crank, forged rods and hyper pistons with do fine for what you are trying to accomplish. If what you are trying to do is towing then you want your power to kick in down low so that your tq band and peak is lower in the rpm range. For this you will probably end up needing to go with around 9.0-9.4:1 compression, since you shouldn't be turning a bunch of rpm you wont need a large runner head. I would say a head with a runner volume in the area of 270cc-295cc would do just fine. Then as far as an intake goes, my personal favorite is the performer rpm intake. then I would go with a carb like the Holley 770 vac sec on up to the holley 870 vac sec carb. As for a cam I would look at a hyd roller cam (flat tappets are next to obsolite now a days) with around 225-235° @ .050, a cam with this kind of duration in a 489-496 should start it's power band around 1400-1500rpm, it wont want to be reved up past around 5400rpm and it will make goobs of tq around 2500rpm.
I would estimate a set up like this in a 489-496 to make around 450-500hp but with around 600-625tq