The timing cover front mount is for the 216 and early 235. A 250 has side mounts. You'll want to use a type of mount that allows the engine's weight to bear straight down on the tops of the frame rails similar to an old flathead Ford. The original upper mounts for a 250 will try to push the frame rails apart. Not a good thing. Try Buffalo Enterprises link on here or Chassis Engineering in Iowa for the correct mount. You'll also need a rear crossmember under the trans. Simply put, you want the engine and trans supported in the same locations as it was in the GMC truck.
