Here is what I ended up doing with mine when I put a 250 in my 66 convertible. The engine mount was the most time-consuming issue to resolve. In the end, I used the stock perches (the part that attaches to the frame), the stock mount/isolator pieces that attach to the 200 engine, and a couple of pieces of heavy steel to act as lowering blocks.
I drilled a hole in each of the isolator brackets between the holes that were already in there. I also drilled 4 holes in the steel spacer, using the stock isolator holes as a template. Then I welded a bolt to the spacer on the side that would be facing the engine.
Now I attached the spacer bar to the bottom hole on the block using a button head bolt to clear the isolator. Next attach the isolator assembly to the stud created by welding the bolt to the spacer bar. Thread a bolt through the middle hole of the isolator/spacer into the top mounting hole in the block. Then bolt the top hole of the isolator assembly to the top hole of the spacer bar.
EDIT TO ADD: I'm using the stock large log head, the stock Carter RBS carb, and a stock air cleaner from a 71 Mustang 250 I6. I sourced the air cleaner from eBay, and you should be able to use a unit from a 70-73 Mustang, Maverick or Comet with the 4.1 L engine. It all fits nicely under the stock hood with room to spare. The oil pan also clears the crossmember and the steering linkage, but I did have to space my front swaybar "U" clamps down a bit to clear everything.