Consider this -

Instead of welding or bolting the top to the base try tacking 4 pieces of angle iron to the bottom of your top inside the top frame of the base.

Place them so you can slide the top to one side or another without upsetting the balance.

This will give you extra inch or two of overhang when needed for clamping.

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l --------- l
l l l l
l l l l
l l l l
l -------- l
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