80W gear oil is almost the same viscosity as 25W engine oil. The gear oil will be fine in your tranny, as will any good engine oil. How often you change it is much more important than what you put in it.
How you break it in depends on what kind of piston you use. Cast pistons don't need much more than a few minutes of easy running and then you are good to go. Forged pistons require a series of heat/cool cycles to break them in properly.
You can use atf type f if needs be, its cheap but u will have to change it frequently, although this should be done with all oils to make sure they did not shear and are in the best condition for your tranny.