Howdy Travis,
Sounds like you've got a bit of a problem! The balancer on a 235 is pressed on (often with a big hammer) and shouldn't have any movement. The key serves as an alignment guide and so that it won't rotate on the crank snout. You shouldn't have any "play" at all!!! The good news is that you've discovered it before it came off and damaged the fan and radiator and the bad news is that there's no quick and easy way to fix it.
The fix: The very simplest fix, if there's no damage to the keyway (unlikely) is to drill and tap the crankshaft and install a Small Block Chevy Harmonic balancer bolt/washer to tighten up the balancer. However, this requires pulling the radiator and possibly some front sheetmetal to gain access. If your balancer outer ring is broken loose, or your keyway wallowed out, it's time for a replacement balancer.
Drilling the crank for the SBC bolt is not as scary as it sounds and is well worth the effort. By drilling and tapping the crank, you can "draw" the balancer down tight and insure it won't come off until you remove the bolt and use a puller.
You're fortunate to have found it before it came off....although you can't detect a wobble it is obviously loose on the snout and your going to have to undertake a labor-intensive repair. On the brighter side....you aren't stranded along the highway with a lot of damaged, expensive parts, and still have to follow the same procedure.
Sorry that I didn't have some better news for you...
Dave
BTW....you'll want to change out the timing cover seal while you've got it all broken down.
