What are your plans for this car? Restoration to original, daily driver, hot rod? The fact that the car does not have the original motor and if you have found it to be a big-block car, the value of it will not be anywhere near as high as if it was complete with the original motor.
I have seen running, streetable early Sharks for around $3000 to $5000. Once you start pricing replacement items, your costs may rapidly approach that of just buying a running car in decent condition. for expample, expect to pay around $1000 for replacing the basic interior components like carpet, seats (foam and upholstery) and door panels.
I would suggest a GM crate motor as a powerplant; they are reasonable in price, come complete except for carb and brackets, and you get a GM warranty. The ZZ4 motor is an excellent choice for a 350 cubic inch motor and any one of the bigblock crate motors (454, 502) will provide lots of low-end torque and serious HP. Complete motors like these go from $3400 to $5000 or so.
Doing a frame off project will require a serious influx of cash. Things like suspension, steering, tranny and differential work can add up quickly. Even thing like weatherstripping can get very expensive on Vettes. Certain areas of the frame on Shark cars like the corner just in front of the rear wheel well opening are prone to rust so inspect the frame carefully and also look for accident damage.
Some owners of old Vettes sitting in their barns or covered with a tarp in the back yard know how much their car is worth. Sometime you just have to ask what they will take for it and go from there. One other thing to do is to make sure you can get a clear title to the car. Try to get the last registration and a license plate number. Fighting the DMV is no fun when you tell them you just bought this old Vette and you don't have a title to it.
Good Luck!!
