It's hard to stay original, at least it was for me, if it were a Shelby, HiPo, GT 390 I'd stay original. I decided to mod my '67 Coupe partly because staying original would not make the car as safe to operate as I could upgrade it to.
Examples of stock to mod preferences I have:
Safety:
nonpower > power brakes
lap belts > 3pt. belts
drum brakes > disc brakes
solid steering shaft > collapseable column
stock short seats > highback seats w/headrest
Drivetrain mods:
2bbl. > 4bbl.
C4
trans. > AOD
Appearance:
14" wheels w/hubcaps > 15" TT D's
Comfort:
heat only > A/C and heat
AM radio > CD/iPod/ Satellite stereo
dash speaker > 6 speakers
Ride:
stock > Shelby drop
stock springs> 1" drop 560lb.
stock leafs > 4 1/2 leaf mideyes
stock perches > roller perches
stock nongas shocks > gas shocks
As you can see this can go on indefinately and depends a lot on how you willl use the car, mine is a daily driver with ambitions of seeing some autocross, 1/4mi., etc. I wouldn't feel as safe without the disc brakes, 3pt. belts, etc. and wouldn't drive it daily without those upgrades.
If you do any mods, even the safety ones your car is no longer original so once you mod, Pandoras box is open...
Enjoy YOUR car!
Jon