I have a 1970 chevelle ss with vin # 136370A174951. can anyone decipher what engine it came with: 396, or 454?

There is no way of telling which engine it came with or even if it is a real SS unless the engine sn# matches the cars vin sn# if it has not restamped. The only other way is to find the build sheet. 1969-1971 is virtually impossibe to tell unless it is a Canadian built car which it might be.

Nope! Only in 72 or above could you tell what engine came in it from the factory from the VIN. You need to find the build sheet or POP.
And starting in 1969 you could not tell from the VIN if it was an SS or not.

I would look for items such as the steering shaft (bolted or welded?), a kick down switch or bracket for a switch on gas pedal, Rear control arm braces, 12 bolt or 10 bolt rear end, a/c or not, motor currently installed, trans currently installed, all should give you some idea as to what motor the car came with. Definately pull the front and rear seats and search for a build sheet also.

Ditto on the VIN of the car being stamped on the block - assuming you're lucky enough to find a matching numbers Chevelle, 69-72. In '72 only, if the VIN indicates a 454 engine, the Chevelle/El Camino has to be a SS since the 454 could only be had with the SS option. A 350-2, 350-4, and even the 402 (396) could be ordered with or without the SS option in 72.

The 136xx series Malibu/El Camino only indicates a V8 engine.