I do have to say that in my past I worked on and owned quite a few Ford flatheads. They are for sure not the best starting cars in the world with the 6V system - probably only just better then an old MoPar 6 flathead. Then let it get cold and/or damp......

Positive ground - and they do have to be clean and shiny, a well built starting motor - one that the end bushing or bearing(depends on the vintage) runs the armature true along with all four bushes being long enough so the (good/strong) springs hold them to the commutator hard enough - and that comm has to be clean, and true with no road grime, oil or mica showing. Then, you do have to have reasonably good compression with good valves plus a carb that is correct. These engines turn very slow in comparison to current engines so need every bit of help they can get. The best thing you can do is a 12V conversion but use a very good 6V starter - the windings are way heavy enough to handle the voltage increase though you may eventually see some pitting on the comm.