The easiest swap is a late ('54 and later) 235 or a 261. These will be a direct drop in replacement, using a front motor mount plate and short shaft water pump. A 292 would take the same amount of work as a V-8 swap (side mounts and crossmember, different tranny, etc.).
-Tim
'51 Chevy 3104 w/'56 235, 848 Head, HEI Dizzy, Corvette Cam, Split Manifold and Dual Carter YF's
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