Others have run two head gaskets stacked together to get the needed thickness. Different thickness gaskets can be used on each side.
If you use either method you will have to double check your pushrod length.
None of this is ideal but short of tearing down your short block it may be your only crutch.
Another thought is to measure the combustion chamber depth of your heads to see if one is deeper than the other. Also may be able to mill the chambers on one head a little deeper.
If this wasn't my daily driver, I would forget all of the above and have the high side pistons milled, or start over decking the block and buying new pistons.