What brand/type of brass blocking rings did you use?
What oil is the trans filled with?
My experience with Toploaders is small, rebuilt maybe a dozen or so over the years.... I have learned a few things...
The brass blocking rings must have slots cut front to rear in the inner coned surface. I'm not talking about the fine inner circumference grooves, but fewer and wider slots machined F to R at right angles to the fine grooves.
Some aftermarket rings don't have the slots... I believe the OEM rings all had slots.
Those slots let the oil "escape" when the blocking ring is pushed towards the coned synchro surface of the gear, during gear change.
If there are no slots, the blocking ring does not "grab" the cone of the gear, but kinda "floats" on a film of oil trapped between the ring and the cone. Thus it grinds cuz the ring can't do its job to synchronize the speed difference between the slider and the gear.
Too thick oil can cause this too.
As an experiment you could run some thinner oil such as ATF just to see if the grind goes away. The ATF won't hurt the trans in the short term.
I even ran ATF in my Toploader for years with no problems. Some say gear and bearing life "might" be less with the ATF, but I never had any problems.
Many newer manual trannys also use ATF instead of gear oil.
I have also had good luck with very lightly "scuffing" the cone on the gear with scotchbrite.