I've had my '99 Sport 3.5L for about 2-1/2 mo now. Been working out the bugs little by little. Not long after I bought it I noticed a slight popping in the exhaust tone somewhere up front but figured it was just a small leak at a flange or something.
After reading that exhaust manifold cracks are common in some of the later models I decided to have a look.

The PS manifold is severely cracked (possibly a complete fracture) between the rear and middle cylinder.

I assume it's not going to cause any additional problems to drive it like this for a few weeks while I line up a replacement, but I'm curious - I've been getting TERRIBLE mileage - I've seen it below 13 a few times. Could a crack that size effect mileage noticably?
It does so-so if it's 100% highway. We got right about 17 ONCE.

I'm not certain what parts interchange. By mitsupart's description I can use any 3.5 OR 3.0 manifold '97-03. Sound right?