Welcome to the Team!
The 427 BBC installed in the Impala was the 390 horse Passenger variant, not the High Performance (with rectangular port heads and a solid tappet cam). You could only get the 425 horse version in the 427 SS impala versus an Impala SS with a 427 engine.
The SS 427 was separate model and the hood alone is worth nearly $12,000. !969 was the last year Chevy offered an SS 427 Impala and there were very few made as every one who wanted a high performance big block was buying them in lighter cars than the big heavy Impala.
You or a prospective buyer can easily pull one of your valve covers. If it says PASS under the valve cover it is a 390 horse 427, or a 350 horse 396. If it says HiPerf then the engine could be worth up to $5000 to some one who wants one for their Corvette restoration project.
You can tell if that is the born with motor as starting in 1967, the car's partial VIN was stamped onto the front ID pad of the engine under the alternator. The ID pad also will have the application code that will tell you the engine displacement, whether it had a four barrel or two barrel, or what tranny was bolted to the back of it when it was built, along with the build date.