Since aquiring my 72 Mach1 I've been going through the same dilema. To start with I've found a very straight car (other than some minor mods and the last owners fascination with chopping the loom here there and everywhere to ad switches).
All numbers match, original colour and interior. 'Plan 1' was to keep it as straight as I found it. This lasted till recently and I have since gone to 'plan 2' and purchased a Mach1 hood to replace the standard hood. Also have aquired front and rear spoilers to add to a previously non spoiler car. I've also decided to change the colour from metallic pewter to Grabber Blue. With this the iterior will change from the stock red to blue and white.
You would think I wanted to end it there
However in my workshop I have a spare 4 cam LT5 ZR1 motor (I also have owned a 1990 ZR1 for the last 13 years)
My two favourite cars are the 71 -73 shape and the 90 ZR1, with the 71-73 edging it. My two favourite engines are the 351 Cleveland and the Lotus engineered LT5 4 cam motor. Both are very pleasing to the eye and to me the LT5 just edges it because of its versatility, unmodified power band and fuel economy.
So just to take stock
Short term aim is body and interior change. Body is at a local paint shop being prepared back to a bare metal job. (all stock interior parts will be stored) The stock 302 with 3 speed are being rebuilt, engine being acid dipped as we speak.
Long term aim is to fit the LT5 and 6 speed, however the realistic time scale for this will be in the next 3-5 years. (again parts like engine and trans will go into storage).
I really did start with good intentions, however you only live once and I want to build my dream combination over the next few years.
Hope I haven't confused you any more