I am replacing the tub in a basement bath, the drain was 'slid into a 1.5" cast hole in the floor. Even with the floor. How do I stub out this to attach a trap/ drain

Are you sure that the cast iron pipe is not already trapped? If this is a replacement thgere was likley a trap in place in the poured concrete, you just can't see it. If there is no trap you should be getting sewer gas coming from the drain right now, do you smell any? Current codes would likely call for a 2" drain, so check the code in your area.

Bal has a valid point. There should be a trap and raiser under the floor. To check if there's a trap simply shove a stick into the hole. If it comes back with a few inshes of water on it then you're trapped. Assuming that you have a trap and the 1 1/2" hole is the cast iron raiser I would, (if you have the room to do it), couple on to the raise with a shielded coupling, (see image) and convert to PVC to connect with the new tub.
But, let's say you don't have the room to install a coupling. In that case I would slip the tub waste into the 1 1/2" hole and caulk it in with JB Weld. This might not make a inspector happy but it'll work. Good luck, tom