First off, if you want decent gas mileage, you arent going to get it. I had a carb set up and my best day was 12-14 mpg. I have the EFI now and still on the highway, my best is in the neighborhood of 16-18 mpg. A 302/5.0 is not fuel efficient. That is one of the consequences that we EB lovers have to suffer through. Secondly, if you dont want to touch the engine because of a recent rebuild, then you need to swap out gears in the axels. For that you are looking in the range of $3000 for someone else to do it. That would also include having a locker installed in both ends. The big problem you are going to run into if you are doing mud running is that you probably have a D30 up front and in the mud you will break it sooner or later. So that brings up the issue of a stronger front axel. Go spend the money on a Dana 44 for the front end. Basically, it snowballs from here because once you get the stronger front end you will have to upgrade the rear because you probably have the small bearing rear end with 28 spline axels and will need to do something about that.
All in all, your axel set up is probably not ideal for mud. You have the small bearing 9" rear end and the smaller and much weaker D30 front end. Save up some money and buy a big bearing 9" with 4:11 gears and get a D44 for the front end and put 4:10 in that. You should also think about doing lockers in either end or both because you dont want to spend the money later down the road to set up either pumpkin again if you do decide to put a locker in. All in all, if you have about 3 grand laying around do the axels first and right because if you dont, you will spend the money fixing and replacing parts for the weaker parts that you currently have.
As far as the tranny goes, put a floor shifter in. I would recommend against a hurst speed shifter because they tend to get stuck. Personal experience there with my first EB. Stick with the 3-speed because you can't jam gears in a 4 speed like you can with the 3.
