" It only has the 350 motor in it which wasn't a very powerful engine. My question is this: when I have the motor rebuilt, if needed, does it hurt the value of the car putting superior internals in the motor for hp and torque gains as long as I keep the block a matching # block? Not looking to build a 600+ hp motor, but more than the original 260 hp or whatever the 350's were would be nice. Maybe 330-400ho range. Input?"

I 'aint no expert, but...

That 350, stock, could be good for 325 HP as built at the factory (2 barrel...4 barrel?). It can fry some tires, depending on your rear end. I don't think new internals will decay the value.