I had a head off of a 6.9 with the precups missing so I thought I would go out and just buy one of the chevy diamond precups and try it. The chevy pre cups are too small to fit in the head correctly, they do have a much larger opening in them. If I ever get the chance I may see if a machine shop can port the 6.9/7.3 opening to match the chevy.
Yes; you can "port" the precup "snout", but not more than .020 to .030 all the way around it, anymore will weaken it too much. For measurement of the port before grinding, take two large rubber erasure's and carve them to fit the port opening, that's your base measurement. On one of them, wrap tape around it to increase the size by .020, do the same on the other, but to .030, use a micrometer to measure, that will be the max grind out size. Thirty thousand's may not seem like a lot, but it's actually opening it up by .120 thousandths volume. A 1/4" carbide end mill in a die grinder is a good tool to do that. Should only take a week or two to do all 8. Have fun.
