Re: Spur of the Moment Trip, Mt.-Id.-Wy. Tips needed...
Your best chance at bull trout this time of the year will be on the South Fork Flathead in the Bob Marshall Wilderness. The river headwaters in the wilderness and flows unimpeded to the N until it reaches Hungry Horse Dam. So everything above the damn is as it has always been: Bull Trout, Cutthroat, and mountain whitefish. You can target bulls there and catch quite a few. The challenge is getting there.
If you don't want to try to arrange a pack trip on short notice, you could go to the town of Columbia Falls where the dam in located and drive to the wilderness boundary then hike upstream as far as you want to. Also, on your back South you could stop at Flathead Lake and fish for Lakers, Flathead is the largest natural lake W of the Mississippi and unfortunately full of Lakers. Kill as many as you can.
I do not know of any Golden Trout lakes.
Sounds like a blast! Have fun!