Look on the back of the engine behind the passenger side cylinder head. There is a number cast into the block.
There are a few numbers for a 69 400. 9790071 is one
9792506 is for the RA IV
9792968 is the RA V
The first one is more common , the second two...not so much
A 9790079 is a 350
Look back there for the numbers.
Fact is stranger than Fiction