I have no dog in this fight but I'll chime in.
You have a "running" 250 that's already installed but you don't say how well it runs. You say you have another vehicle for towing so I assume this means you don't need more power. Assuming the six runs acceptably, leave it, dress it up and deal with its freeway manners via the rearend ratio.
The 350 is only a block? To me it's a slam dunk that the six will be cheaper and it's inarguable that the 250 will be more unique than a 350 belly button.