Great pics! Good to see you were able to get up close and personal with it. What are you going to do now..buy it and restore it? Apparently it has been a BBC Vette all along. 390 Horse 427. Didn't they option them with Tri-Power or was that the 400 HP 427...hmmmm. IIRC the 390 was a single carb, iron intake.
Regarding 'Rat' motors..all Big Block Chevy's are 'Rat' motors (I don't like the term MOTOR, but what the heck..a shop teacher years ago would rant and say a MOTOR is electric and an ENGINE is gas! hehehehe). A 'Mouse' motor is a Small Block Chevy.