probably the only person on this site whos done a Duraspark conversion....
i highly recommend it
its very easy and can be done in less than a day
and if you do your research first and scout out the cheapest parts it can be done for less than $200
through some good fortune and knowing people i did mine for less than $120
one thing i recommend is NOT using a holley 350
just use a non feedback 2150 motocraft carb off an early 80's 302
it will be more than enough for the 2.8 unless yours is built up
got most of my info from here: http://www.therangerstation.com/tech...Duraspark.html
now for your mileage issue...
there is a way to hack your converter and lock it with a toggle switch....which is also explained in the link....it may help your gas mileage a bit until you can get it replaced
personally, with my durasparked 2.8 in my 4x4 using winter gas and mostly city driving in and out of 4wd and 2wd
the best i've been able to squeeze out so far has been 14 mpg
i don't even think thats accurate since i have different size tires and my odometer works when it wants to
the first few weeks after i finished building it, i did get 10 but it was mostly because i couldn't keep my foot out of it
i'm willing to bet the converter is your main issue....if its slipping your just wasting gas while it spools up