Dave McClellan (may he RIP) once dyno'ed his 455 stage1 before and after putting it in his Buick. It lost just over 25% in the car.
Kinda depends on auto or stick, and if an auto, how big/how much trans and converter.
Depends who you talk to, but I believe 10-15% for a stick, 20% for a glide or maybe a 904, 25% for a turbo400, 727, C6.
Rearend makes a difference too. 9" eats more power than a dana/8 3/4/ 12 bolt.