Wow - stunning car originally ordered in these colors, with the L72. Having the original car shipper document is gold.
Sounds like neither the engine (CE block) or trans are original, but for the right price I'd love to own it. Looks like it has been recently spray bombed underneath, which is a bit of a bummer.
Strange the the M20 RPO is designed "4 SPD TRANS & TACH"
Link to 1966 L72 Impala SS auction
168376S170853