MISERABLE Mileage.............

As for the timing, Mine is set at 7*BTDC and runs quite well, at 10* it had some pinging issues at partial throttle. The books (depending on what one and what year) will tell you anything from 0* to 5* BTDC for the slant. I agree with tophat, check the vacuum advance pod on the dizzy, if you aren't getting enough advance at cruise, your mileage will suck big time.
and for the rebuild, buy a kit from the local parts place and do it yourself, it will take about an afternoon including cleaning time. The only things to be careful of are the 2 check balls, one under the accel pump and one in the venturi's, search for the bbd manual at the slantsix.org site.
It sounds like you are on the right track, replace the usual maint stuff and check mileage again, then on to the harder stuff :)