what size screws to attach the filter on a 727?

I don't know the exact TPI or length, but they are the same thread as the valvebody screws and about an inch and a quarter/half long.
Use a valvebody screw for thread pitch reference and chk the depth of the hole with a pocket screwdriver or something similar and deduct a quarter inch off the total depth of the hole for the length.

Assuming you can't get a set of OEM of course.