Will a ls1 intake manifold fit on 1998 4.6L Northstar engine?

No, an LS1 intake manifold will absolutely not fit on a 1998 4.6L Northstar engine. They are completely different engines with different bolt patterns, intake port configurations, and overall designs. The LS1 is a small-block Chevrolet V8, while the Northstar is a Cadillac V8 with a significantly different architecture.