Welcome. I have a 99 Silverado myself. Just got it 2 mos ago.
She only has 112K so she is barely broken in. I have added an Airaid Jr Cold Air intake, a Performance Chip from GRacing and installed a Scan Gauge ll. I also did a tune with HyperTech power programmer lll.
Not sure how much gain on HPwr but the MPG is 15 city and 19 hwy.