I know the 4.3 liter V-6 is basically a 350 short a couple of cylinders, but don't know about the 3.8, thought it was a completely different set up. I know of someone that runs one in a 40 chevy, the guy said it's more like the turbo grand national motor.
The 4.3 does have the same bore and stroke as the 350, not sure what parts would interchange, pistons, rods and maybe the timing chain setup. Did here you can make em go pretty good though, they make some pretty good torque numbers.