stingc

I have a car with the more or less the same engine, but just after fuel became unleaded (74 vette). The gas mileage depends on how the car is setup (is the engine stock?), how its geared, how good of condition its in. I've seen 350's (355 is common for a rebuilt 350) get anywhere from 5 to 25 mpg, with 12 about average. If that's a concern, you can improve it slightly for some performance loss. The engines in these cars are fairly reliable though. Its everything that's attached to them that you should watch out for. As everyone else has said, its best to just have someone who knows about cars to look at it.