'99 Silverado 1500 runs very badly

Pulling onto a crossroad I felt a definite change in power delivery. Not
necessarily less, but certainly different. Also noticed a clicking/rattling noise
that changed with RPMs while at a red light. Truck runs incredibly rough
when it runs, which isn't often. Smells like fuel when it runs and sounds like
an exhaust leak near passenger firewall. I checked the plugs and all are
decent. Truck only starts on occasion now. It has 105K on the odo. Manual
trans on a 4.8 Vortec.