Don't be thinking right off that any 455 is going to be a screamer, if it is really a 74, performance could be very lame, check this site, you need to find the block ID under the passenger head in the front, it would be a WZ or YH etc etc, then look at the center exhaust port on either head and try to figure out what that says, then look around the distributer for the year code, the long set of numbers would be when it was made and the large 2 numbers should say 74 if it is, meaning that is what year car it is meant for, now look at this chart. You will find that all 74 455's (except the SD) were 250 hp with 8-1 comp and the 067 cam, if you want to stay low comp for todays poor fuel and get some real boost go with the 068 cam, it will leap that motor, probably to over 300hp, if you find the heads so so, either rebuild or go with edelbrock which are based on the RAIV heads in either D or round and with the 068 cam you would have a 455 superduty.

let us know the numbers, I'm dying to find out

if that is a Y8 block with 16 heads it is the 74 SuperDuty and a rare find,

http://www.classicalpontiac.com/cgi-bin/cp/global/engine.cgi