It will bolt right up without any major modifications. Your C4 or small block bellhousing will work fine. The 289 motor mounts will work, also. I had a 4 speed in my '67, but I found a place in Mass. that made headers for this swap. The Cleveland is probably 100lbs heavier than the 289, so the nose will sit a little lower. You can reduce the engine's weight by installing an aluminum intake and headers. A larger radiator will be needed. One for a big block '67-'70 Mustang will work. You can also get a good aftermarket aluminun one. You may have to relocate the ps pump and a/c compressor if you have them.
I put a 351 c in a 67 fastback, use the standard small block motor mounts, use a 1970 mustang radiator and make your own mounts. I went with a 4 speed, and all the linkage is a an easy swap. auto swap is easy too. 351's make great power. canted valves kick ***. I went 11.2 compression ratio, 292 hyd. cam, 13 lb alum flywheel, and am running about 500 horse. U cant go wrong, just get a good block and dont bore more than .030.
lets assume that your 1967 mustang is in good to excelent condition. Origional mustangs sell for between 30 to 50 thousand dollars. yank the 289 and drop in a cleveland and you will have a very fast mustang worth about 5000 dollars. good luck, and please destroy your origional mustang so mine might be worth 75000 dollars thank you.