fourfourto just so you know there were not any 403 motors made in 80's thats when the oil embargo was going on and they pushed manufacturers to stop producing large motors, in most cases we havent seen a motor larger than 350 since the 70's, the 80's killed heavy horsepower. I promise there were NEVER any 403's in any 80 or 81 Firebird Trans Am. Even though this is not a Trans Am site just wanted to offer more info if you care.
http://www.iwaynet.net/~gl&lisk/transam.html I dont know much but I know TA's Ive had 6 of them including a 1969 Trans Am, kicking myself in the A$$ daily for letting it go, sold it in 86 for 3400.00 now its worth about 30k!