Ah, the never ending tale of "fuse woes" in a 911. I did the flat fuse conversion on my SC a little over a year ago. Since then, no blown fuses and no electrical problems! Here is a picture of the rebuilt fuse box in my '80 SC coupe.
The fuse holders came from Norway (Thanks Ove) and are rated for fuses up to 20 amps. The only existing circuit that exceeded the 20 amp limit is the fuse that supplies power for the front condenser fan AND the evaporator fan. Since there was room for 24 of the new fuses (vs 21 originals) I simply put the two fans on separate fuses. By mounting the fuse holders in an aluminum bracket that I made, the original fuse cover will still fit. You will probably have to extend a couple of wires and be sure to use the very best connectors that you can find. On the connectors that are attached to major power leads, I soldered and crimped to make certain of a good connection.
Good Luck!
Fred Cook
'80 911SC coupe