Most likely not the issue but just for grins check your chain for wear/stretch. Lay a ruler or tape measure along it and see if the links all match up nicely with the 1/2" marks on the ruler. The longer of a run you can check the easier it will be to tell. If a link is more than 1/4" or so out of step with the ruler over 6" then replace the chain, but if the chain lines up within 1/8" over 12" then continue on with the derailleur search.

Also see if you can remove the rear wheel and get a metric measurement between the inside faces of the rear dropouts. Some of the above comments coupled with the bike having a 6-speed on the rear makes me curious.

Nice looking bike and those shifters look interesting.