Jim,
Headlights shouldn't be fused, relayed perhaps. Alternators are a direct wired setup. Cooling fans are usually run through a relay with the main power feed inline-fused with a 35-amp American-rated blade fuse. Perhaps you should ping Dan Masters for his book on TR6 wiring. His details about auxiliary electrics and additional power sources would certainly apply to the TR4 in most ways. Dan is listed as danmas on the forum and he spells out most every conceivable issue related to the Triumph wiring, especially the issues of fuse and wire ratings.
Bill