The only shows that true originality matter are concourse and MCA thouroghbred (sp?).
Most shows are divided into two classes, original and modified. You can normally stay in original if you have minor modifications and the car still "appears stock". Aftermarket carb, exhaust, electronic ignition, minor chrome in the engine compartment, different paint color/interior color are some of the changes you'll still see acceptable here. Sometimes an engine swap is acceptable if it was offered with the car, like a 390 swapped into place where a 289 was originally but the 390 is detailed to look "original" to the car, for example.
The modified classes are for cars with heavy engine mods, fancy paint/graphics, modified body, fancy rims, roll cages, blowers, rack and pinion.....generally moderate to major changes to the original car.
These are general rules but it really depends on who's putting the show on and how it's classes are laid out.
Cruises are just laid back gatherings without classes for the most part and any car, in any condition can be on display here. You'll see everything for highly detailed cars to weekend drivers to everyday drivers to works in progress that run and drive but have rust, mismatched body panels, primer, grungy engine compartments, incomplete/worn interior, ect.
If you plan on driving the car alot and want some upgrades then do it and enjoy it and just show the car and not worry about it.
Question about "showing" my Mach... for the people that show their cars 
