Dollar per horse power, you are better off with a small block 350. It will be way cheaper, have more power, idle smooth, and if you shop around a decent warranty can be had. Not saying you can't make a 250 run decent, but it will cost quite a bit of money, drivability, and fuel mileage. If you really want a 6 cylinder with more power, look for a 292, it should bolt in with a little motor mount swapping. Just so you know, 292 are really hard on gas mileage.
You are better off with a different transmission, just a carl suggested, and rebuild the engine as is.
