I had the same issue when I split the cases on my tecate;
1. Go over those cases with a fine tooth comb to make sure you have got every bolt out, check behind any gears, pins or bearings that you may still have on the cases. you don't want to be trying to lever cases apart that are still bolted together.
2. make or obtain a puller assy that pushes against the crank and attaches itself to one side of the case.
3. Apply a little heat to the area around where the crank bearings sit. Be very careful don't melt the cases, they are old aluminum. just a little heat for expansion.
4. be paitent, they will come apart. They're just old and cranky (geddit "Cranky")
5. I initialy got mine apart with two screw drivers and a soft mallet, but i recommend you to use a puller instead to avoid scoring the case halves as they have to make a proper seal once reassembled
My toys 85 KXT250A2, 85 ATC250R, 85 Tri-z 250, 06 LT-R450, 04 YFZ450S, 07 125 typhoon x 3, 06 FPV GT.