It would depend a bit on whether you are in a snow/salt prone area. Where I live we see snow about once every four to six years. Salt is what we put on popcorn. Any rust here is from a vehicle that "immigrated" here from elsewhere. I installed factory buckets from a 1994 ECSB in my 1988 SCSB by doing just as you said. I drilled and bolted - no caulk. I have zero rust. Yes, I know I should seal up the holes a bit, but my intention is to weld a reinforcement patch and a weld nut on the underside of my floorpan for each hole. I just haven't gotten around to doing that yet. I do check each bolt/hole periodically. Still no rust. If you need to seal, I would suggest some direct-to-metal primer, followed by paint, and then as an extra precaution, seam-sealer. It's about $20 per tube at the local auto body supply shop.