The flywheel- or in your case- it is really called a "flex plate" is bolted to the crankshaft, and it is what is used to bolt the torque converter to the engine. The starter's ring gear is also welded to the flex plate. If in the rare instance you do have a manual transmission, then the flywheel is used to transmit power from the engine through the clutch/input shaft of the transmission. In either case, it is located between the engine and the transmission.
Answer Chevy engines are totally different from Pontiac engines. you'll need an adapter plate to fit it to a Pontiac transmission, assuming you're using the firebird tranny. check www.jegs.com they have the adapter plates there.OK...I don't see where the Pontiac engine comes into play here. In '86 it's a Chevy engine in that Firebird. As far as I know, the flywheel from the 305 will bolt right up to the 350. Use the 305 flywheel, clutch and pressure plate and possibly the starter. Chevy had two different diameter flywheels/flexplates.