If decarb is seldom or never done with as old as that 98 is I would assume the buildup on top of cylinders was pretty thick which would have increased the compression and the tune was slanted because of that and now tweak of tune is needed.

Not sure if the sparkplugs used are same heat range as stock but in any case I would change them but go at least one step colder then stock like a NGK-TR6

Sorry but if decrab is not done and due to LS1s eating some oil it would not take long for carbon to built up again and another reason to use water/meth injection to keep cylinders and piston tops clean

Also if orginal knock sensors I would consider replacing them with the newer design then what 98s had.

As to MPG, again I would do good testrun with OBD scanner recording, analyze that data and tweak tune but I would do that after those sparkplugs are changed