Could you just order a 400 CI in a BASE 1971 Firbird?
 

I'm following a car that the seller says HAS a 400 in it and says they cannot find any stampings to prove it's a 400.

It think it's a 350, and I can adjust my offer accordingly.

Any help is appreciated.

Please disregard my post. The seller finally said the OE engine was replaced with a 400......since the stampings didn't match the VIN.

But the seller couldn't tell me what year the 400 block was.........so I'm moving on.