I little bit of logic would help make it more believable. Of course it won't fool an experienced motorhead.

A multi two barrel intake would help. Three deuces, or maybe something like the old Manafree four deuce model. I believe the flathead had three exhaust pipes per side, so some simple custom exhaust would help, or at least non-ramshorn factory manifolds with the single pipe per side.. Paint anything that you don't want noticed flat black or gloss black to hide it. And maybe you could distract from the details by doing the valve cover lettering in a light or bright color, along with some chrome air cleaners, and colored plug wires.

These things won't fool the experts, but you would be making the Chevy heritage less obvious.