Doing the same job on my 75 sovereign
currently, only a few steps ahead of you. What I did was grind what was left of the bolts off flush with an angle grinder. To reinstall the plywood and channel, I am moving the bolt locations just an inch or so left or right, also adding some more bolts. I found at Ace Hdwre some 3/8 self-tapping lag bolts which are working great. Drilling 5/16 pilot holes and letting the screws cut their own threads into the hold down plate.
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