If you did not buy/install the ground strap(s)/cables, you cannot be sure they are 2 gauge or bigger (1, 0, 00 gauge), which is needed in a 6v system. All my 6v trucks had 12v ground straps when I bought them (in 1969, 1971, 1997, 2005, 2007).
Even the reproduction truck parts sellers had the wrong gauge power&ground cables for 6v.
Jim Carter now sells 2 gauge power cables, but their ground cables might still be 4 gauge.
Chevs of the 40s now sell 1 gauge power cables, but their ground cable might still be 4 gauge.
Starting of the engine significantly improved after installing larger-gauge ground straps/cables (I now use 00 ground cables from a tractor supply store, or a welding shop or big truck shop/store).
As strongly suggested above, clean (until grease&rust free, and shiney) all contact/connection posts/surfaces and coat them with a conductive copper-impregnated anti-oxidant grease (available in a small tube at most auto parts stores). Starting will be significantly improved, and cable-ends will last much longer.
