The ring gear lock tool is available ok ebay. You remove the couple of long bolts into the oil pan closest to the access hole into the bell housing. Slide in the locking tool so it engages the teeth of the ring gear. Use the oil pan bolts or provided bolts to hold the tool in place. There's a similar tool that slides in place of the starter. It's an option if you don't mind removing the starter or if you're working on the engine with the oil pan off. In that case just wedge a crank counterweight.

Sixto
87 300D