Attitude is everything as some have pointed out,Modifying dashes is generally OK as long as the padded section remains basically stock as this is a safety standard and they reinforce that even more so than simple compliance stuff,\.
Early vehicles only had to comply with standards applicable at the time so Webers on a 1200 are OK,But if you drop in a CA18 then you have to comply to Later Standards applicable to the Engine Year,That is a trap many are not aware of.

Therefore do your research carefully and modify sensible and try to find the increasingly scarce engineer that will sign off on mods so you are covered,An earlier car means less hassles as it has less rules to comply with.

Basic roadworthiness such as minimum height and legal tyres no sharp bits sticking out etc will save you much grief down the track but you can still have a decent nice ride.

Just remember no cars that belong in a Transformeres movie.