Truck looks real nice for $1300...I think that is more than reasonable. I just purchased a 77 short bed, 6 cylinder auto...paid $200 more for what appears to be one in a little worse shape than this one.
It does have 79/80 headlight bezels on it though...you'll
have to fix that.
Clifford Performance deals with 6 cylinder engines and performance, but your 250 will not have much aftermarket performance support as it is fairly undesireable.