After I bought my '85 300CD-T, I discovered the same situation you describe. I fixed the problem by replacing the rubber seal ring that fits onto the bolt. The old ring had worn out and was allowing fuel to seep by. I would recommend not applying any excess torque to the bolt. The whole fuel filter assembly was designed to function properly with the proper torque on this bolt. If the bolt head is leaking fuel, even with the correct torque, the problem is elsewhere.
One other thing: Some bolts use two seal rings, some use one. You'll have to pull yours, check it out, and then acquire what you need. This worked for me.