Question about rear gear choice

[nightmare]

I know, I know, another thread about which gear to use with a 700R4. I know most have said to go with a 3.73, but that's with a 28" tire. I've got a 30" tire, and am wondering if a 3.42 wouldn't be a better choice. Right now, I've got 3.08's, and do not like how the gas mileage dropped like a rock.

Does having a longbed adversely affect mileage? I'm just wondering how people are getting 18 to 20 mpg in these trucks.

I'm not expecting that type of mileage, just something more than the 12 I used to get with my TH400.

I've got a longbed with a stock 350, will have a fairly fresh 700R4, and am wanting to choose the right gearing for this combo. I would also like input as to which ring and pinion company (Richmond, Precision, etc.) would be better to use, or which to stay away from. Any thoughts?