Thinking of buying this:

1988 Silverado (K3500), new 502 cu. inch crate engine with <35,000 miles on it, manual transmission, Edelbrock carb with manual choke, garaged.

Would $6000 be a fair price?

 

Well the way I look at it is, if the buyer feels the price is worth what he/she is looking at and test driving, then it's worth it.
It doesn't matter what the sell feels it's worth, it's what the buyer feels its worth.
There are too many variables to say the truck you listed is worth $6k...was the engine installed correctly? Are there wires going to no where? Any rust? Condition of the trans, clutch, interior...etc, etc, etc...only the buyer can and should make this decision.
A question to ask yourself is how much is $6k worth to you?