666 what I'm saying is chevy started building 396-375 horse power cars in the same time frame as your car and the first few were 4P coded. Being you said it was a race car most of its life it might have started life as a 396-375. Is the rad. a small block( 21") and do the heater hoses enter the fire wall behind the head. If so it more than likley is a small block car