Looks like you got a great boat anchor. Even if it was a bigger engine, a tall deck big block won't fit under the hood of a C3 unless you use an aftermarket hood.
Also, other than maybe the crankshaft, there's not a lot inside an HD truck engine that's useful in a performance application. The pistons are really heavy, four-ring units. The cylinder heads are not a good performance choice.
I'd take it back and tell the guy who sold it what happens to folks who misrepresent 366es and 454s.