Exhaust Manifold Gasket?

I have a 90 Lumina 3.1. I have exhaust gas leaking in through my heater. I just need weatherstripping under my hood to fix it but have other questions. I noticed where my flexpipe meets the manifold..it is loose. I can shove a screwdriver in there. I noticed only two loose bolts as well. Isn't there 3? What symptoms would cause a bad manifold gasket? I have had a ticking sound that increases with speed for about 3 years now and it is not suspension or tire related (or timing either). Would this decrease my MPG's and what other damage would this do to my car if I drove it for about 3 months like this? BTW..the exhaust from flexpipe back is new.