It all depends on the character I would say… Who is this guy? Does he know about cars or is he the usual rube who thinks that driving such model will make people think he is a car guy.
The modifications will vary depending upon the guys background.
What would a mechanic drive? If it was his car and he knows his trade? Again its a character thing… Some would drive something bone stock as it rolled off the Chevy assembly line…every nut and bolt zinc plated with the right color…the factory mufflers, hubcaps etc… Others wouldn’t… again…depends.
Is he the type where everything must be as it should be? Or is he a bit more devious where looks arent always what they seem. You have an open canvas here…use it.
Or is this guy the dark mysterious type with a complicated former life that people are trying to figure out…the car is a great vehicle for piquing peoples interest in who this guy is or was in years past if he had an LS7 under the hood…a manual transmission…maybe even a blower or turbo in the mix…with an overdrive on the trans (tho not needed if a 6sp trans is used)… the overdrive unit would be another interesting add on, to older transmissions that only had 4 gears (it shows the car knowledge again…and also marks in time when he acquired the vehicle because he needed to add an overdrive when 6sp manuals were not available it marks a time period of when he built this vehicle so to speak) Or is he techno savvy…with a stout automatic with 8 gears and paddle shift buttons hidden in the wheel? Sooo many options… ones that could turn what looks like an old relic…into a Mad Max road burner capable of 200mph…
Bigger brakes, Modern tires…wheels that look stock but arent… especially “Steelies” wheels that look like crappy stamped steel assys…but are actually widened wheels meant for more serious modern rubber (and he better have some rubber with a blown LS7)… a modest 1.5 inch suspension drop with firmer rate springs… or even air bags… oh boy… I need another classic car to play with I think…
Also…for those clandestine nights when he actually may need to evade someone? A brake light shutoff switch…which allows the headlights to operate normally but blacks out the rear of the car to obfuscate which way Mr Dectecto went… that sort of thing? If you wanted to go James Bond…an Oil sprayer under the rear bumper to send people off the road behind him… LOL… Im getting carried away.
A modern GPS tracking system would also be nice… a hidden camera in the dash… Kind of like a “Bait Car” so he can see which A hole tried to steal his car or record conversations in the front seat for future review and case solving details that he stumbles upon during said review…the Ah hah moment in some case perhaps…courtesy of his car…
An alarm system with a smartphone link…for remote start, door unlock…etc…shaved door handles and door poppers that only work with his Fob or remote… To thwart those same A holes that tried to steal his bait car prior.
I could go on and on… The mods depend on who is behind the wheel…and what you want those mods to say about him.