Like everything in life, there's a right away and a wrong way to customize. The right way could be fitting a modern motor and transmission to a classic car body. That way you get the economy and reliability you want in a daily driver with the looks of a classic.
The wrong way is bolting an Easy Rods '50s Ford lookalike kit to a 1995 Ford Thunderbird. It's the sort of kit you buy when you have a) money for spending and b) no taste.