71 DEMON - slant6/3 speed manual to RB/727 - Questions!

oh man... if what you guys are saying is true, i may have made a seriously careless mistake here!!! the guy i got the New Yorker from told me it was a 440, i opened the hood, saw that front mounted distributor, took it for a test drive and wrapped the deal up right then and there. holy smokes... has anyone ever gotten that sinking feeling in their stomach?