Not sure on the exhaust air injection holes but studs on the rockers and vertical intake bolts along with the magnum style head bolts lean towards magnum.
There is no "lean". The Magnum head is the only factory head for the small block that has stud mounted rockers. Period.
Vertical intake bolts can be added to an LA head if someone had a reason to run a Magnum intake on one. A simple matter of drill and threading holes. So stud mounted rockers are the visual indicator here, the casting number will be a double check.
As far as getting a flywheel, your problems have just begun, lol. You may, or may not, have and LA rotating assembly, or a Magnum one. We already know someone put a Magnum top end on, it's not out of the realm of possibility that it also has a Magnum rotating assembly, which means different flywheel balance. Heck, I guess it could be internally balanced as well.
Casting numbers on the crank and rods will tell you your answer on LA or Magnum, added balance material in the crank will answer the internally balanced or not question. LA rods have a wider small end that will not fit in a Magnum piston without machine work.
