The new one is a GM 10SI alternator.
1) Typically that mounting diameter is 3/8" bolt or stud. I've drilled them out for non standard applications- Ford diesel, John Deere, 262 Cummins, etc.
2) Yes. the fan pulls air through the alternator, fresh air enters the back. Alternators don't care about which way they rotate.
3) Old alternator has a 'keyed on' wire, as well as a charge wire. New one just has a charge wire. Eliminate the 'keyed on' wire.
Dan'l