Why not look for a stock intake/exhaust manifold to fit your 250 from a Junk Yard?
One issue with the header you have and all of the header choices you have been given, is there is no heat riser. The heat riser redirects exhaust gasses to the bottom of the intake (not into the intake where the gas vapor is, but into a jacket surrounding the bottom) to help vaporize the liquid gas when the engine is cold. As the engine heats up the riser valve shuts directing the exhaust away from the intake and out the exhaust pipe. Some of the headers have ports to direct some of the exhaust to the intake which make the engines run better as they heat up, but there is no valve to shut that heat off as the engine heats up. Once the engine is at operating temperature, the intake is sufficiently heated and does not require additional heat from the exhaust.
Engines just run better with a stock heat riser.
