This is what has worked for me, and it is they way I have timed my gasser street racers too.
I get the pump just loose enough to move it, then I advance it until the truck starts missing (something other than dead smooth).
This all has to be done when the engine is hot.
At this point the truck will pop when revved in neutral.
Then I road test. Any popping or misfiring at all and I stop and retard the pump.
Once I get it all smoothed out I know I have the timing perfect, and the truck runs like a beast.
I have not tried this with a timing spacer and I assume once the pump is timed at idle with the spacer it will need to be retarded a bit to get the top end smoothed out.
I have found that at higher altitudes pump (and spark) timing needs more initial and the same total as sea level, in other words less of a spread.
I would be willing to bet that my truck would be sluggish at low RPM with the timing spacer at my altitude.