How thin is it? industry standard back then was about 3/8" thick out side skin...you may be making more work for your self then is necessary then simply just bonding the core to the existing transom...that being said if you feel it necessary to lay down the extra glass then you could get a couple straight 2/4's placed on edge and counter sink some holes so you can get some screws into the old skin to keep it flat then do like you said and grind them flush with the inside skin...maybe put a little wax on then just to be sure they will back out easily...you may find that after removing the braces that you still have a bow and then you can do the whole thing over again when adding the core...
strick
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"I always wanted to piss in the Rhine" (General George Patton upon entering Germany)
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