For stress relieving, you want to be at least 100F below the critical temperature, which is generally 1100F to 1200F. Once reaching temperature, it is only important that you let it cool slowly. Production shops want to rush that part, because they have to make money. We're just looking to make a few quality parts.
My electric furnace is almost done, and it will have temperature control. But , if I were doing it with propane, I'd pack the forge with as much of the steel as I could get in there and heat it to cherry red and hold it there for an hour. Then cover the forge with as much insulation as I could and go to bed.
That is the plan for all the thick parts I'm working on. (The fun part will be case hardening the base, after the machining!!)
Failure! One problem was my rookie mistake. I did not have enough weight on the flask and floated the cope. Duh. Nevertheless, the bulky casting pattern is not idea for yellow brass and would have significant shrink defects anyway. I think I need to thin out the web to about 3/8" and add a shrink bob. That should do it.
I don't know how thick you made these, but it still might be usable all the same. I could potentially even just lathe it out into some sort of relief if it wasn't too bad.