Shadowlakes, there's gonna be a variety of answers and no one will be true, or they will all be true.
The "worth" or "value" of the car is entirely subjective to who's being asked. Since the car is yours, the first person I'd be concerned about it
you. Is it worth more to you with the replacement engine or the original engine?
Are you getting this car just to cherish it and baby it for the next owner? Or do you plan on keeping this car for yourself and your purposes? If you have little intention of selling the car, then it only matters what it's worth to you.
If you think that later on you are going to want to sell the car, then it's just a matter of finding this buyer who wants what you have. It will be more attractive to some with a higher output engine (the replacement). It will be more attractive to others that want a factory original. No matter what, there will be someone to buy your car should that time come. In the meantime,
you should enjoy the car and not worry too much about the future. Unless you are in the speculation business and/or turn over cars on a regular basis, just enjoy whatcha got!
With that being said...
To add a little something to your question of "what about dealer installed options?" There are special examples of dealer modified cars that typically go for more, such as cars from the Yenko family dealerships.
In your particular case, since it was a 1975 car and the replacement engine is not a 'genuine' LT1 engine, it's really nothing more than a crate engine ordered by and installed by a dealership. Nothing really special there. You could do the same thing today. Order a base C6 and then order the LS7 Z06 engine via the dealer. Just because they do the install doesn't make your car a Z06. Later on, it will be worth more to someone looking for a car that already has more power than stock, but it won't be worth more to someone looking for an actual factory Z06.
Overall, the resale or collector value of your car is only worth what someone is willing to pay for it.
