Here's a pic of the type of starter brace I'm talking about
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This particular example is on a small block, but similar braces were used on big blocks as well. Finding one can be a bit tricky since they were commonly thrown away when someone replaced a starter and didn't figure it was necessary. You might still be able to get one from a GM dealer or restoration parts supplier. Or look for one at a junkyard. You could probably even make a homemade one from a small piece of angle iron.
I can see where checking the timing could be a bit difficult in your situation. And unless something was recently changed that would have disturbed the timing, you'll probably be okay with getting the new starter shimmed/braced properly. Although, I suppose you could set the static timing like you'd do after replacing a distributor and then fine tune it with the timing light once it's running.