You're on the right start with the split grade, but may need to persist. Try burning in the highlights with 00 after a lesser exposure with that same filter, just enough to put something in the shadows, rather than dodging during the longer exposure. Then try a test with #5 to find out the amount of exposure you need to just barely get a black (maybe even without a 00 exposure first). Then try that combo. You are right, with a thin neg, dodging with #5 will be touchy. And in the end, the neg may not have enough guts in the shadows to make a good low end image in the print. Sometimes the #5 only exposure tells you something about the separation that is even possible in the shadows.