Concur on the big tire solution. I recently put 16" Dexstar 8 lug wheels on my 1949 3600 (261 and SM420), and shod those wheels with 235/85 R16 radials. That gave me a tire diameter of 31.7", which sounds enormous, but in fact just matches the diameter of the optional 17" wheels offered in 1949 (31.5" from the factory sheet). Tires were only $100 each, but I couldn't find anything less than 10 ply (in the 85 ratio). 50mph generates about 2500 RPM and 2900 RPM yields about 60mph. A big change from the 235/75 R15's that were on before.
