NICE! That stag looks a little less like ass than most. Like a set of factory second/blem discount ivories.
The trick to really stomping on the .357 Mag gas pedal is going to be choosing a bullet that put as much it's weight as possible forward of the case mouth. I gotta do some homework on that for ya. My quick and easy reference for the various brand/model cylinder lengths was LBT's website, but Veral Smith recently passed away after a long string of personal tragedies - health and his shop burning down - so much of that resource has faded.
Elmer Keith's 358429 (nominally 173 grains) was originally designed for the Heavy Duty .38 Special loads. Usually, you have to seat it deep and crimp over the front driving band for its long nose to clear the front of most cylinders when using .357 brass, but a Redhawk might have enough room.
At any rate, I'll spend some time this evening researching to help your SIL pick out a mold.
Edit to add: as for "I already bought a Pre-27", with a Redhawk (or even a GP-100) in the stable, you won't be tempted to run the super high octane aviation gas in your irreplaceable classic car. Ya done good.