You mean replacing the 305 with a 4.3?
About the same power if the 4.3 has the 4 barrel carb.
I would go with a 5.7....more power and will bolt right up without relocating anything like motor mounts,ect...
Will the 4.3 not mount in the same location as the 305? I have a good deal on new engine but want to make sure everthing will work the same. Not really worried about gaining more power.