So this is a topic which has been covered lots of times so please bear with me.
my t is running 12v with a standard 6v starter, has done for several years, I know some people have had lots of differing experiances.
I agree that the wear and breakages could be higher on 12v so if nothing else want to reduce the impact of the inital start up on the car, shes getting older and general fitegue of parts will also become more of problem im sure.
the question - what sort of wire should I run from the battery to the starter to add some resistance, just to soften the flow, all experiances welcome.
Opinions differs, but you can always try a bendix spring as a "resistor" in series with the battery cable, but the best way is likely to get the starter field windings modified from series/parallel wound to fully series wound - but that can wait until you have broken your first bendix spring on the starter & work is inevitable..
(Ok, work on a bad starter isn't inevitable - Ford put a hand crank on every T and it's actually fun and easy to use )
See this 2008 thread for a lot of discussion...
I rewired the starter myself. Soldiering the connections required a red hot pair of pliers.
Over the years I have run quite a few six volt starters on twelve volts. T's, A's, V-8's of various kinds, VW's, Corvair engines mounted to VW transaxles, ect. The best advice I can give, is that it will work just fine, until you have a starting problem. (You don't want to crank the engine any more than you have to).