The muzzle of my SXS was bad so I shortened the barrel.
Looking from the muzzle end, there is a gap between the barrels and rib. 
I was thinking about filling it with low temp silver solder for appearance and to stop any possible barrel separation.
Any thoughts on this?

Detach the barrels and suspend them muzzle down by the locking lug or whatever (the idea is to let them not be muffled by external forces). Tap them over their full length with a piece of wood or a rawhide hammer and listen to the sounds they make. If they ring like a bell then the integrity of the barrel ribs is ok. If you hear a dull thud, then the rib/ribs are compromised and should by rights be re-soldered. If they ring, and that unsightly gap needs to be filled, I have simply mixed some thickened epoxy dyed gray/black and squeegee'd it in. I did that on a German 16 gauge double that got truly hammered over a couple of hot and heavy dove seasons and the "repair" held up fine.

How much did you actually cut off of the muzzle end and do you still have the original chokes?
This in my own opinion is not one of those self imposed attempts to being able to do it right. 
Do a search for this gentleman as he does have video that teaches one on how to do the rib work right.

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Actually nothing is really rocket science. 
Thanks to the Internet now days folks like himself can now keep us from becoming a Parkenfarker and then trying to dump onto someone else.