As many have said here, the little Rockwell can be run somewhat hard if taken care of, sealed VERY well, synthetics and an extra quart doesn't hurt either. I ended up swapping mine out with a 205 out of a Dodge (divorced) because I didn't have the means to go through the 221 at the time. It was starting to leak pretty bad which led to getting hotter than I was comfortable with.
For several years, I towed a 4500# boat + 1500# in the bed in 3-4 hour stretches at 70 - 75. This was behind a mild 396.
Only toward the end did it start making a little more noise than normal. Thus the swap.
Although the 221's design lends it to running hotter than the 205, it is still a good case if set up right and can work quite well for most all jobs. The 205 is a little overkill unless you have a good one sitting around just waiting to be popped in (which I did and even then it didn't just "pop" right in. It was a lot of work).
My 221 is still patiently waiting for some attention under the work bench. Every once in a while it reminds me by jumping out and stubbing my toes for me.
Make every effort you can to get some good bearings into it which will help keep the seals in one piece and then put some thought into the extra quart of oil and better venting. I ran +1 qt for a while with no issues and it made a difference.