78 GP

I found a 1978 GrandPrix (the small GrandPrix) with T-tops, bucket seats, power windows brakes, etc . He did change out the idiot lights and put in the gauges - from other pontiac Everything standard on it - the guy who had it did some work so it looks good, (could use a real professional paint job but from 20 feet it looks good tho - new brakes, shocks, frame is good -- no rust its got 235rx15 all around with cheap mag wheels, - but he did take out the 301 and put in a 73 Pontiac 350 which is stock, but also has the 4 bbl manifold and carb. he just never installed it-- Is this car worth anything?and what should I offer him so as not to insult him ?