“I want to make contrast masks from 6×7 [120 film] color. What film and developer should I use. I can’t afford 4×5 Tmax100”

First: don’t use 4×5 size film. Use 120 film, cut into 3 inch length.

Next: it must be panchromatic film to collect (mask) all the colors on your original without being a separated mask.

Do this:

Use Ilford PanF film. Ilford PQ Universal developer. Arrange the stack as illustrated. Set the enlarger as if you were making an 11×14 print using your suggested lens. Follow the start points indicted in the drawing.

The black paper underneath is more important than you may think. It controls reflected light and flare back toward the cover glass and lens.

PQ developer 1:20 for 3minutes continuous agitation 68degrees F.