The real sin is not contemplating a temporary modern engine transplant to get you mobile, the sin is permanently butchering the vehicle's engine or trans mounts so that a change back to original down the track becomes a major work or even impossible. As pointed out in other posts, it is possible to buy "Bolt-in" conversion kits to fit a Chev V8 in place of the Ford Flathead and these fit without any modifications. These will also work with a Chev V6, but the engine mounts will need a bit of imagination. Unfortunately, the Chev V8 doesn't have the same exhaust beat as the Flatty

and depending on the cam fitted, may not run well with good torque at low revs for parade work!