68 camaro 6 or 8??? Help

Hello all, I just recently purchased a 68 convertible and It has the original 6 engine(230) and I am deciding on replacing it with a crate 350 engine maybe the zz4 engine but I do not want to devalue the car and actually looking to improve its value so if anyone can give me some facts on replacing motor to a new engine or keeping the original, then please leave your comments. All comments appreciated. Thanks The overall condition of the car is a good 3 thanks Hope to hear back