76 GMC Camper Special 350 4 barrel

[truckdriver]

I found a 76 Camper Special with 58,000. Completely stock, the glove box sheet says it has the m40 turbo. I thought it would be a 400 but have been told it would be a 350 turbo. Was considering a crate 383 or 502 high torque. Would the 350 turbo be enough for the 383? Cruising at highway speeds, would the 383 get just a couple of miles per gallon more or would there be a huge difference? Haven't been able to determine my differential ratio yet. How does everyone like the 502HT?