There are many ways to hold a part, and no one correct way for all parts
On my old open knee mill, I used to machine parts like that right on the end of the bar stock. I had enough stock sticking out clear of the vise to do all the profiling except for the 3" flat end. I would usually make some kind of a support post that bolted to the table to support the stock for drilling. So when all operations were complete, then I would pause the program, and add a strap clamp (or use a bolt hole through the part if available) onto the finished end, bearing down onto the support post. Then, do a cutoff operation to finish the long flat cut. Machine the cutoff path a little bit in the rough, and not quite through, perhaps leaving .02 attaching the part to the bar stock. Then, move to the exact finish line and profile finish the end face of the part, leaving the .020 for a final cut. Even then, the tool can want to grab the part and gouge it, so you may need to dream up a fingerstop to prevent such an accident at the end of the final step.
Now in a machining center, its not to handy to have 10 feet of stock hanging out so I would probably go with the 1/2 thick stock method and perhaps make a set of soft jaws to hold the completed part for a final facing operation when it is flipped over.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)