You can look up recommended rim widths for specific tire sizes on Tire Rack's website, http://www.tirerack.com, clicking on "specs" for each mfr's tire model of interest. The same info is in brochures at tire shops. (Note that rim "width" in the tire and wheel industry is the inside dimension between the rim flanges that confine the tire bead, not the extreme edge-to-edge width, which is typically about one inch more.)

Old tire brochures I have give 4 inch width as the minimum for 165 R 15 tires, and 5.5 inch max. Current specs for 185/65-15 tend to show 5 inch min and 6.5 inch max rim width range. Others have replied that they have gotten away with what you propose, but it seems not to be what the tire mfr's recommend.

Wider steel rims can be installed on original center sections. See http://www.stocktonwheel.com/wheel_widening.htm for an account of how this is done.

Check out the "Wheel Swaps/Bolt Pattern" thread of approx Feb 15, 2002 in the Brickboard archives, especially Jim Hekker's post, with website link, for comments on wheel widening alternatives for 120's.

I have been considering the same tire size for my 544, which cannot accept significant extra rim width to the inside (ie, extra "back spacing") because of its suspension front upright's design, proportioned in the 1940's for 16 inch wheel diam. 120's and later seem not to have this problem.