Maybe I'm not reading your post right. If so, I apologize....
You are going to take a portrait, right? Use 135mm and hope to use wide open. ISO is 400. You use yellow filter.
I love taking portrait but the last thing I'd try is try to do that in FULL SUN. Extreme contrast and shadow will be very difficult to manage. Also, I would probably not use yellow filter for portraits. I just don't need it. I'm more inclined to use partial shadow. With it, it exposure should fall fairly reasonable even with Tri-X. Come to think of it, I've done this plenty of times with Mamiya M645Super which topped out at 1/400 or something.
Maybe high contrast portrait is what you'd like to do.... If so, I'd just pick more appropriate speed, rather than try to use Tri-X.