The form/trim die will do a couple of things. It will push the shoulder back almost all way to where it needs to be allowing the final position to be set by the FL sizer. Also it will allow the excess case length to be cut off using a hacksaw. If you have a power trimmer the FT die is not an issue, taking over a 1/4" off using a crank gets tedious really fast. When I had my 357H barrel I would expand the necks first then run through the FT die. Then I would clean up the case mouth on a case trimmer and setting the length right at max, followed by setting the shoulder in the FL die. It takes some trial and error to set the shoulder, you will want to position such that when closing the action will take a smart snap to lock up and allow it to dry fire on an empty case where the hammer strikes and rests against the firing pin. Even though it is a rimmed case it headspaces on the shoulder. Too much and cases are toast within just a few firings. Your friend there likely has the process down pat anyway. It takes time like casting, and like wise the satisfaction of making everything vs buy, load, shoot.
Tell ya what, on second thought, somewhere....... I have a load manual put out by TC specifically for the Contender some 30-40 years ago (OMG....) it describes things better than I. I will look for it, copy and mail the info on the Herrett rounds for you. Has your address changed since we traded bullets? Let me know if you need any other data and I will include it as well.
Back when pistol silhouette was the big thing, some folks liked using 375 Win due to the heavier construction and thicker web and for the reason you mention. I never went that route, 30-30 brass was often free to pick up on the range or 5, maybe 10 cents each and even then 375 was not all that common. Starline is showing $160 for 250 pieces