Hi, have know idea what a 81 spider is but if its an old car then the best and safest was to get in is to use the old "whacking" trick. a lot of older cars can be opened this way ESPECIALLY fiats even up to 98-99. what you need is a similar type key to your own one preferably a worn one, put it in the lock and giggle it up and down whil putting light pressure on it to turn, try not inserting it all the way in and it should unlock the car easy enough.
the reason this works is because the pins in the barrel get worn over time, especially the drivers door, ignition and the boot and tend to get stuck in the open possition so you only have to move them a little and they should line up correctly to open the door. This is a very common method used by theives because it is very quick and doesnt require any tools.,,
if it works for ya, you may want to concider getting an immobiliser.,.,
good luck,.,.