You'll also likely need a better connecting rod for the 250 engine build as well. A stock rod isn't going to be up to the challenge, so more expense there too. Its hard to argue the better technology and abilities of the newer 4200. Its superior in every aspect compared to these older engines all the way down to the cost. Plus, you have more room to grow power wise, while as you said, the 250 is going to pretty much be at the top end of its ability and limits, and this usually means low longevity. So I think the obvious choice is the 4200.
