I guess you'll have to consider how nice of a condition the car is in, plus the extra mechanics that were added. If you planned on getting an unmodified car and doing what is already done to this one anyway, then there's a value to what that car has.
The car has 160k+ miles, but what about the engine and transmission? It originally came from the factory with a 305, not a 350, so that 350 is a transplant. But, was it just an old engine that was dropped in, or was it rebuilt? I would also check the numbers on the block to make sure it is in fact a 350. Some people like to pass a 305 off as a 350 since it's hard to easily tell them apart. And since you said the car is at a dealership, you could also assume they might have their heads up their butt and don't know what they have there.
This coding info will help: http://www.chevy-camaro.com/chevy-ca...oding-help.asp

 

Mileage really doesnt count on this car as nothing appears original. I hate to say it, but where Im from, thats a 4000-4500 dollar car all day long. That engine has several thousands of dollars put into it. I wouldnt try your 'take it or leave it approach'.....the dealer knows you are interested and knows you are playing hardball. He will just tell you someone else is looking at it for asking price, blah blah, you know the deal.

This car sounds like a soild, fast, well built car. Id verify some of the parts, takes it for a ride, and give it a day or so......if yer still interested, make him a reasonable offer.

 

You cant use KBB when considering the mods. An engine can add 5k, a rear end can add 2k. That seems high for an 84 but not if it really has all that stuff done.

That car would be worth every cent of $4500 if what he says is really there. So, if it is clean, the color you want, has wheels you like, and runs great then it is worth $5500 to you. I cant say for certain if I would pay almost 6k since I cant see that actual car. But if you bought a stock car for $3000, you could not have a shop do all that work for anywhere near $3000.