Well, I probably should have done this before drilling the top guide strip, but better late than never. I've clecoed the firewall recess in position to check if it will interfere with the top guide. Since I can't see inside the heater manifold with this in place, I'll mark a line on the recess at the top edge of the heater manifold.

Once unclecoed and setup on the workbench, I can see that it's close, but there's about 1/16 inch of clearance between the firewall recess and the top guide. Whew!

Next, I got out the heater box to check the travel required.

Looks like about 1 3/16 inch travel. So that's what I want to shoot for for the travel of the heater baffle control as well.

Doing a little math, it looks like a bellcrank length of 2 17/32 should work well. Note that's center to center distance.