There are perhaps several levels of "modified". At one end, if you can put it back to how it was, that's pretty conservative. On the other end, when you take the cutting torch to it, you have moved a long ways.
As far as value goes, an old saying is "a car is worth what someone will pay you for it". So one guy might be knocked out by your wire wheels while another is thinking "can I get some original wheels and hubcaps someplace?". George on this site did some great stuff putting new instruments into the area where the radio is - some people would love it while purists might be put off.
In the final analysis, unless you are going to sell it at some point and hope to squeeze every last dime out of the sale, it seems like you should enjoy it all you can and feel good about making it your own personal car, however modified that may be. The only exception to that might be a car that is totally original and near perfect with hardly any mileage (there was one a year ago with 19K that Mark had I recall). Rather than modify that car I would say a person ought to sell it and buy a car with more wear and tear.