I own what I believe to be a 1971 455 not sure thought maybe someone could help me here are some of the numbers i found on the block and heads. On the back of the block by the trans my block has 1231798 cast into it. On the passenger side is GM with a 2 underneath it behind the head. Between cylinders 1-3 are the numbers and letters 40H929221 and between 5-7 are SF 28I. On the passenger side head under the valve cover I found GM with A 16 underneath it on the frontside of the head. On the backside I found a 4 with 1231786 underneath it.
1970 not a 1971. Also you miscopied or mistyped the block number. It is 1231738. SF combined withe VIN suggests it came out of a 1970 Riviera.
1231738 is a '70-'71 block casting, 1231786 is a '70 head. SF indicates a 1970 big car 455...looks like a non-stage 1 1970 455. Devon
Spend a few days scouring this message board...specifically the "search" function at the top. Lots of go-fast parts available. Devon
He is correct but it is a small valve engine. Its output would be less than the 70 Stage one engine. Relative to the 376hp real world stage one it would be about 15 hp less or about 360hp.
