As long as the tennis ball is way bigger than the pipe and there won't be any chance of it getting sucked in, sure, it's safe to do, however, if your main drain is filled with concrete (many people plug them after having issues that are unfixable), then your entire suction side is almost completely shut off (May have a trickle left around the tennis ball fuzz), at which point your pump will start to create quite a bit of suction, leave this going too long and you could cause damage to your 40 yr old plumbing, so my suggestion is:
1. Add some dirt on/around the main drain so suction is very evident right away(you can always vac it back up...)
2. position yourself at the edge of the pool watching the main drain closely and holding the tennis ball in position at the skimmer
3. Get a helper to turn pump on, but not for more than 10 seconds.