What would be the MPG on a 350 CID engine with 290 HP in an 83 Silverado and 700R4 Automatic overdrive ?

I have a 350/290 HP GM Crate engine, I have 700R4 Automatic Overdrive and 3.08gear ratio in the rear and is 2 wheel drive 1983 Silverado. I just cannot seem to find anyone with such a combo and am really curious how many Miles per gallon I would get, because I don't have it running yet and will try to prepare for the impact on gas money because I took out my 305 for this 350/290HP engine