Assuming this is a #3 car, which may be generous, if it is a 2 door, about $3500 to $4000.
This is assuming about $3000 to $3200 for the car
Plus 15% for the 350 motor
Plus 10% for the 4 speed transmission
See the second link to understand the condition code.
With collectible cars, in general, modifications from stock can detract from the value. There are some exceptions of course. See the third link.
The ultimate value of a collectible car is the size of the check that someone is willing to write for it.