I think your 64 Impala number is too low.
Edmunds shows them around $3000.
I know I bought a new strippo Chevy II (Nova) for $2300 in 67 and back then prices weren’t going up very fast.
My Mom bought a new 66 Impala for around $3K.
That is possible. It was a long time ago. I'm pretty sure about the $4000 figure for the Corvette.