When I did mine, I took a straight edge across the bell and measured to a conveter bolt pad with the converter all the way on the input snout. On the flex side I used a dial caliper thru a bolt hole to the seperator plate. Subtract a from from b, and voila. Too much pull out and the converter snout lugs won't engage the pump gears, and, well, bad news bears. Too little and you wipe a thrust bearing and, again, no bueno.
What flex are you running btw? Some are just not right because "aftermarket". I'm using a tci one (with a stock v6 trans and converter to get it running) and it was right in range. There also wasn't much excess thread on the converter studs so don't know how well too big of a spacer would work.