I thinkthe best thing would be to install an Ait Fuel gage (AFR gage) These run a couple hundred bucks. Innovative motorsports is very popular and they have an active forum. This will help you with the tuning. I get 15-16 mpg with the blower so you should be able to get the same or better....this is on 87 gas too. Also install a vacuum gage. These are a great load sensor....low vac means low mpg.
Next you will have to ditch the gears. Hoosier makes a Pro street tire in the 31 inch range. I have these and they have 18k miles on them now. They work great in the rain...even in the light snow last week. With a good suspension they will hook quite well too. These run about $700 a pair and work tubless. I'd gear for about 2500 rpm at 60 mph +/- a little what ever it takes to get this.
An OD trans is shaky at best....JMHO I cruise with over 150 guys and there are a lot of OD's in the group. There are a lot of them in the shop for repairs too. You will spend $1500-1800 for anything that will hold up and even that is shaky at best. If you had factory EFI the story might be better. Bottom line is any OD is going to be relatively expensive. I like my T-350 without a kickdown. I just manually drop it down a gear if I really want to charge on the highway otherwise I just step on it.