Generally, there would only be a market for a clone/replica/tribute where the genuine article is rare and/or expensive. Genuine V6 capris are getting harder to find, but values are low compared with more desirable cars, and also generally lower than the median income in Australia ($45k).
If you'd value the car at $15k (if it were genuine) then I'd say reduce that by ~ 30%. However, given it is a 1600 GT, that must be worth something extra? Also, condition and modifications/parts (e.g. better gearbox, improved motor) would add further value again. The trend I see with classic/muscle cars is that genuine and original cars is where the real value is.
The main issue I'd have is the car being represented as genuine, but this shouldn't be something you worry about as you're being honest at the time of sale. But with the extra V6 bits added to the body, it's easier for some scumbag down the line to misrepresent the car. Making that worse is that there is still a lot of heresay and conjecture around what genuine V6s had/how they were built. I know of a handful of cars that were sold as genuine V6s that are clearly not, and the owners either don't know or don't want to know. I've been stung in the past, which is the main reason I harp on about it.
Have you got any pictures, that might help put a value on it?
Cheers
Jamie