Is the hole size more like 1" x 3"? A cassette is wider than 2".
Is this the only hole in the dash? No factory radio hole? Your new deck should fit into the factory radio hole. May need a bracket, if not supplied.
For the deck to work correctly, it should be mounted correctly. I'd recommend a hole large enough to accept the mounting bracket. CD's skip if the spinner isn't mounted firmly; and skipping is a MAJOR pain.
Or, if your goal is a stock '66, how about patching the hole closed and goin' with an under-dash or glove box mount? That is, if there was no stock radio from the factory. You could patch with fiberglass, or welding a plate and some bondo. Course, then you gotta paint the dash.