Hi Spoomah and welcome!
It's no problem to use the L-500XL with your stock neck pickup, but there will be a little volume mismatch as XL is fairly loud and with a totally different tone spectrum than your 490T. Bill's L-500R is a tremendously good neck pickup, though, if you'll ever want to replace the neck pickup, too. ;)
Your SG is almost certainly outfitted with 500k pots, so the swap should be a breeze.
The L-500 is a bit narrower than a Gibson humbucker, so there shouldn't be any reason to modify the pickguard.
I suppose you've got a black guard, so if you order a black XL the small gap will be more or less invisible for all but picky, eagle-eyed ones :b