If you are replacing all those bearings, etc., it will be much less expensive to buy an R&P installation kit which has all the stuff in it to begin with - including the crush washer. The crush washer might be reusable but then again it might not. Since they come in the kit you won't have to wonder if it will, or won't work. You shouldn't be using an impact to set the crush; you can easily overshoot the desired pinion pre-load and also potentially damage the new pinion bearings you just installed. Using an impact to take it apart is OK provided you plan to toss the bearings anyhow. The repeated impacts can brinnell and ruin the bearings.
If it all comes within torque specs using the old washer then all is well. Problem is, you don't know if it will be that way until AFTER you try to set it up. If its not OK then you have to remove the outer pinion bearing which requires ruining the pinion oil seal and potentially damaging the outer pinion bearing as well. A pinion oil seal by itself costs more than a crush washer - why risk using the old washer?
The kits have seals, bearings, shims, marking compound, etc., and some even have the outer axle bearings and seals. I think I mentioned it before but the early V8 axles use a special (smaller) axle seal that is not in the kits but available at AutoZone.
The 8" uses a slightly smaller crush washer than the 9". The individual crush washers you buy at the local parts store are all for the 9" models. Since I bought the Ratech 8" installation kit I can tell you that it has the correct crush washer for an 8". I don't know about the other brands. The 9" crush will likely work but since you have to crush it about twice as far as an 8" one it may well take more torque to crush.
Since you are replacing the actual differential with a Track-Loc its somewhat irrelevant but... After having bought and installed the parts I can confirm that the internal parts kit (differential gears and shaft) for the 2-pinion, open 8" diff are identical to those for the 9" except the roll pin that retains the spider gear shaft is a different size. No one claims to sell those parts for an 8" but the 9" kits they all sell use the same parts except for supplying a larger pin.